Why is MT preferred for OT translation instead of LXX?

Reasons include:

1. Hebrew is the original language of these scriptures in almost all cases. Greek was not used by Jews until many years after the traditional dates of the writing of the last OT scriptures. While the MT is less ancient than the LXX, a direct translation is generally preferred to a "translation of a translation."

2. Comparison of the MT to the Dead Sea Scrolls reveals surprisingly little variation from these very ancient scriptures, which often predate Jesus - and also predate all extant copies of books found in the LXX. So the OP's supposition is not well founded - that the MT may contain intentional "hiding" of prophecies supporting the identity of Jesus as the Messiah. However, it is indeed true that in some cases (not all) the DSS align more closely with the LXX than the MT. 1

3. In terms of ancient manuscripts, the LXX is not consistent within itself. Remember the LXX is not one book but many, and the earliest complete collection is a Christian one. 2 In other words, it was preserved by Christians, not Jews. In fact, it may be the case that Christian tampering with the LXX - a point the OP does not consider - is more likely than Jewish tampering with the MT. 3

4. The fact that early Christians used the LXX results from their being mostly Greek speakers - an "accident" of history rather than reason to prefer Greek to Hebrew.

5. In principle, to prefer the LXX over the Masoretic is similar to preferring the Vulgate (Latin) New Testament to the Greek versions. Most of the Western fathers in fact used the Vulgate, and for a millennium it was the official bible of the Western church. 4 But few today argue that it would be better to translate the NT from Latin than from Greek.

Of course, good translations utilize both Greek and Hebrew texts, noting variations in manuscript traditions - especially when these have potential theological significance. But the MT is a better starting point.

Great question!

There are some misconceptions about the Septuagint (LXX) and the Masoretic (MT). These are not two definitive texts. Rather, they are both texts that are derived from earlier text sources that have been lost to time.

In fact, there are many texts of the LXX -- and most of them date to the 4th and 5th centuries AD (CE). So, when people say "the Septuagint says thus and thus," what they really mean is that the surviving copies or translations of the Septuagint say thus and thus.

Yet, the text sources that we have for the LXX are largely in agreement with one another. The problem, of course, is that we don't have a very large assortment of ancient or early LXX texts. Moreover, it is vital to remember the very purpose of the LXX -- to translate the Hebrew Bible into Greek.

With this in mind, the seventy (i.e., LXX) -- or seventy-two -- translators based their work upon an even earlier Hebrew manuscript source. Unfortunately, we don't have this text for comparison to either LXX or MT manuscripts.

However, this brings up a explanation to the MT part of your question: The New Testament writers didn't use the MT because the MT did not yet exist.

It may be accurate to say that the New Testament writers used the LXX. However, it may also be accurate that they used earlier or different LXX texts or even earlier Hebrew texts. We simply don't know which texts they were quoting because they didn't cite the manuscripts that they used.

While it is clear that some of the things that were quoted by Jesus and the apostles diverges from the MT, it is possible that they were either referencing/quoting from earlier Hebrew texts, an earlier LXX, other Greek translations (there were at least several), or earlier Aramaic texts. From a theological perspective, it is possible that they were referencing or quoting Scripture through inspiration of the Holy Spirit.

Regardless, there is a point that the LXX and MT don't always agree. However, they are both taken from earlier sources. In fact, it's possible that they are both taken from the same primary source lineage (albeit centuries apart from one another).

I wouldn't say that one text is superior to the other. Rather, I would say that translators -- using the oldest text sources -- tend to consult multiple texts in order to find textual agreement. If two or three sources agree, it may add some credibility to a particular set of source texts and the effort to translate a Biblical manuscript accurately. This is why footnotes are very helpful.

Assumptions

This question contains some misconceptions which I want to address:

LXX predates MT by over a 1000 years (though one could argue about the power of rabbinic tradition)

This is a very common belief but a closer look at what we mean by "MT" and "LXX" can clear things up a bit. First, the original Septuagint which was famously translated by a council of 70/72 Jewish scholars for Ptolemy II in ~280 BCE was only the first five books of the Torah (see the Letter of Aristeas). The rest of the Hebrew bible was translated by various translators over the subsequent centuries, including after the advent of Christianity.

What most people are talking about when they say "LXX" is the Hexaplaric LXX and that dates to the mid-3rd century CE. However this manuscript no longer exists. In fact, the earliest LXX document would be the 4th century CE Codex Vaticanus which is actually an incomplete version of the Hexaplaric LXX. Therefore you must rely on the 5th century Greek manuscripts to complete the "LXX". So the earliest complete LXX-type text is 5th century CE. The MT dates to early 11th century. So, the MT is actually only 550-600 years younger than the LXX that you're referring to.

LXX was translated under supervision of Sanhedrin and high priest (who obviously weren't there by the time of MT)

This claim is referencing the traditional legend of the Septuagint’s origin, but it is not historically verified and is likely false or exaggerated in this particular form. The primary source for this story is the Letter of Aristeas (200's BCE), a pseudepigraphal account describing how King Ptolemy II of Egypt invited Jewish scholars to translate the Torah into Greek. But this translation was only the first five books of the Hebrew bible. There is no record of the high priest or Sanhedrin overseeing the rest of the Greek translations of the other Hebrew bible books.

Scribes were known to alter the Hebrew scripture just after the time of Jesus to hide fulfilled prophecies of Jesus (although it could be debated if these alterations have made their way to MT)

This is another common accusation but entirely without merit. For example, Justin Martyr (~150 AD) famously accused Jewish leaders of expunging certain phrases from Scripture (e.g. he believed Psalm 96:10 originally read "The Lord reigns from the wood [of the cross]", which he alleged Jews removed. However, modern scholarship finds such charges dubious. In the case of Psalm 96:10, no Hebrew manuscript has that phrase; instead, it appears that some Christian copies of the Greek LXX had added "from the wood" as a Christological gloss (it shows up in later LXX manuscripts and Old Latin copies), and Justin assumed the Jews "removed" it. This and similar cases suggest the early Christian polemic of Jewish tampering was often based on misunderstandings or reliance on variant texts that were not original. Another early Father, Irenaeus, accused Jews of altering passages like Isaiah 7:14 ("young woman" vs "virgin"), but again, all evidence shows "young woman" ('almah) was the original Hebrew (a complete copy of Isaiah was found in the Dead Sea Scrolls which agrees with the MT Isaiah 7:14 reading).

Furthermore, there are selected passages where the MT is more Christological than the LXX, such as the famous Isaiah 9:6 passage. The MT reads Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace. The LXX changes these titles to the less grand Angel of Great Counsel. This is not the only example where the LXX seems to read in a less Christological way, so clearly the MT editors did not flinch in preserving verses that would seem inconvenient to them.

Dead Sea Scrolls tend to align more often with LXX than MT

This claim is essentially completely false. Unfortunately fully addressing it would require far more detail than is feasible for a StackExchange answer but this summary from Wikipedia lays out the relevant facts nicely:

[Most] of the Qumran fragments can be classified as being closer to the Masoretic text than to any other text group that has survived. According to Lawrence Schiffman, 60% can be classed as being of proto-Masoretic type, and a further 20% Qumran style with bases in proto-Masoretic texts, compared to 5% proto-Samaritan type, 5% Septuagintal type, and 10% non-aligned. Joseph Fitzmyer noted the following regarding the findings at Qumran Cave 4 in particular: "Such ancient recensional forms of Old Testament books bear witness to an unsuspected textual diversity that once existed; these texts merit far greater study and attention than they have been accorded till now. Thus, the differences in the Septuagint are no longer considered the result of a poor or tendentious attempt to translate the Hebrew into the Greek; rather they testify to a different pre-Christian form of the Hebrew text".

So up to 80% of DSS biblical manuscripts are essentially in the Masoretic family. In contrast, only about 5% of the fragments could be classified as “proto-Septuagint” or Septuagintal in type (meaning their Hebrew underlying text agrees with the known LXX against the MT).

Summary

Modern biblical scholarship recognizes the Septuagint as an essential witness to the ancient text and how it was understood, but also affirms that the Masoretic Text is a highly accurate preservation of the Hebrew Bible. There is no conspiracy of late changes; most differences arose naturally in the text's early transmission. The consensus is that each source (LXX, MT, Dead Sea Scrolls) must be examined to recover the most original text.

It is only the consonants in the MT that are the original language. The vowel markings were added later. Sometimes the LXX shows a meaning of different vowels supplied with the consonants. Texts with a textual critical apparatus will show theses LXX variations in the critical apparatus. For example, BHS Exodus 19:3:

The critical apparatus for this text references particular texts of the LXX which imply a variation from the MT to get their translation.

I think first we need to realize that LXX -- nor MT, for that matter -- is not a monolithic "thing." Before really the Sixtine edition in the 16th century, you can't just hold "the LXX" in a bound copy for study. "The LXX" looked differently at different times and places, and that as a term can really refer to just about any ancient Greek translation of the OT, even translations of the MT (the so-called "revised editions" of Aquila, Symmachus, etc.)!

In the earliest sources (letter of Aristeas, Philo, and Josephus), LXX refers to just a translation of the Pentateuch alone. The first author to cite a non-Penteteuchal book as "LXX" is Justin Martyr, which makes you think what authors a generation before, most critically the NT authors, considered the LXX to be (most likely, a translation of the Penteteuch). What we think of as LXX really doesn't begin to pop up until you get the great Codices, and even then these Codices contain books their authors almost certainly didn't consider Scripture, like the Shepherd, so it's hard to tell if inclusion in one of these Codices meant a book was part of "the LXX."

The reason MT is taken as the starting point of all translations or text critical efforts of the OT is because it's basically what we've got. You need to base the OT on a Hebrew text, since we know that it was originally written in Hebrew. And MT is the oldest complete Hebrew source. And it's proved itself to be ancient through the great agreement it has with many of the Dead Sea Scrolls outside of Qumran. And, to be sure, MT is a good text, but realistically and logistically, it wouldn't even make sense to begin with a different text.

But we need to realize that the Hebrew tradition is similar, and MT is not perfect in all its parts. We can in apologetic contexts overestimate how awesome MT is (which, to be sure, it is awesome); but what we really need, I think, is an eclectic approach which takes into account all sources. So we need to retain MT in many places just because of how good it is, but rely more on the LXX for books which are "bad" or rather secondary in MT (like Samuel or Jeremiah).

Why is the Septuagint often preferred over the Masoretic text?

There are several reasons, historical, philological and theological:

A. Antiquity of the LXX text

• The translation of the Septuagint was made between the 3rd–1st centuries BC, that is, it is based on Hebrew manuscripts older than those used for the Masoretic Text (10th–11th centuries AD as manuscripts).
• In many places the LXX agrees more with the findings of the Dead Sea Scrolls than with the Masoretic, showing that it preserves older variants of the Jewish tradition.

B. Use of the LXX in the ancient Church

• The LXX was the Old Testament of the Apostles and the early Church.

• Most biblical passages quoted by the authors of the New Testament agree with the LXX and not with the Masoretic.

• Thus, for the patristic and liturgical tradition of the Church, the LXX has theological authority.

C. Philological reasons

• In some books (e.g. Daniel, Jeremiah, Samuel–Kings) the text of the LXX appears more extensive, more coherent or with significantly different readings.

• In some cases it is considered that the LXX renders an earlier stage of the text, while the Masoretic has received later editing, cuts or adaptations.

What is the tradition of the two texts?

A. Tradition of the Septuagint (LXX)

• Greek translation from Hebrew and Aramaic texts of the era of the 3rd–1st centuries BC

• The translation is not uniform; different books have different styles and origins.

• It spread to Greek-speaking Judaism and was fully adopted by the Church.

• Main manuscripts: Codex Vaticanus, Sinaiticus, Alexandrinus (4th–5th centuries).

B. Masoretic Text (MT) Tradition

• Formed by the Masoretes (6th–10th centuries AD), who stabilized the Hebrew text, added vowels, accent marks, and notation.

• It represents a specific rabbinic tradition.

• The most important manuscripts: Codex Leningrad (1008 AD), Codex Aleppo (10th century).

Is it necessary to correlate the two texts for critical restoration?

Yes — absolutely.

In modern biblical philology:

• The LXX and the MT are two autonomous but complementary testimonies of the Old Testament.

Critical restoration requires:

• The Masoretic as the most complete and stable Hebrew text,

• The LXX as testimony to an earlier phase of Jewish tradition,

• The Dead Sea Scrolls as a point of control and comparison,

• Other ancient translations (Syriac Peshitta, Latin Vulgate, Targum, etc.).

Example:

• In the books of Samuel–Kings, the LXX often preserves a more coherent and less corrupted text than the MT.
• In Jeremiah, the LXX is about 1/7 shorter than the MT and seems to represent an older version.

Summary Conclusion

• The LXX is preferred when it appears to reflect an older or more
  coherent tradition.

• The MT remains the primary Hebrew witness and the base text in philological editions.

• The combination of the two is necessary for the best possible critical restoration of the ancient text.