TED's answer is fantastic; it covers the root causes of the problems. I'd like to add one more cause, specifically, US military and economic intervention in Central and South America have acted to keep these countries poor.

Some examples, working from South to North:

1. US backed Chilean coup of 1973

2. US Backing for Argentina Dirty War

3. US Backing for 1964 Brazil coup d'etat

4. Bolivia’s US-backed president, Gonzalo Sanchez de Lozada

5. Separation of Panama from Colombia

6. US Backed Venezuelan Coup

7. US backed military rulers of El Salvador

8. US Backed Guatemalan Coup

9. US - Mexico War

10. US administration of Cuba, Haiti, Dominican Republic

These are just a few examples of US intervention; most people from South America are well aware that the US has been meddling in their affairs for decades, generally to the benefit of the US and not to the common welfare of South American people.

The US generally backs rightist oligarchy/aristocracy types against popular rebellions, so honestly, we can't blame the US 100% for these problems; really the South/Central American ruling classes refusing to share power and wealth with the people is the primary cause.

However, without US help, it is unlikely that popular reform would have been as suppressed for as long.

TED's answer is fantastic; it covers the root causes of the problems. I'd like to add one more cause, specifically, US military and economic intervention in Central and South America have acted to keep these countries poor. Edit: The new answer by Luís Henrique provides a ton of detail.

Some examples, working from South to North:

1. US backed Chilean coup of 1973

2. US Backing for Argentina Dirty War

3. US Backing for 1964 Brazil coup d'etat

4. Bolivia’s US-backed president, Gonzalo Sanchez de Lozada

5. Separation of Panama from Colombia

6. US Backed Venezuelan Coup

7. US backed military rulers of El Salvador

8. US Backed Guatemalan Coup

9. US - Mexico War

10. US administration of Cuba, Haiti, Dominican Republic

These are just a few examples of US intervention; most people from South America are well aware that the US has been meddling in their affairs for decades, generally to the benefit of the US and not to the common welfare of South American people.

The US generally backs rightist oligarchy/aristocracy types against popular rebellions, so honestly, we can't blame the US 100% for these problems; really the South/Central American ruling classes refusing to share power and wealth with the people is the primary cause.

However, without US help, it is unlikely that popular reform would have been as suppressed for as long.