Monospaced fonts make it much easier to line up code.

This is especially true when working with a team; everyone in the team can use different fonts, and as long as they're all monospaced, everything will line up. Similarly, if a single person uses many different development tools, everything will line up if they're all monospaced. If they weren't all monospaced, you'd have to make sure that they all use the same font, and if you're developing on two platforms, that can be difficult.

In fact, some development tools only support monospaced fonts.

Another reason is that monospaced fonts tend to have more distinct characters. Compare lIio0 to lIio0, and you'll see what I mean. They also make it much easier to count whitespace.

Monospaced fonts make it much easier to line up code.

This is especially true when working with a team; everyone in the team can use different fonts, and as long as they're all monospaced, everything will line up. Similarly, if a single person uses many different development tools, everything will line up if they're all monospaced. If they weren't all monospaced, you'd have to make sure that they all use the same font, and if you're developing on two platforms, that can be difficult.

In fact, some development tools only support monospaced fonts.

Another reason is that monospaced fonts tend to have more distinct characters. Compare lIiO0 to lIiO0, and you'll see what I mean. They also make it much easier to count whitespace.