Typos feel way harder to spot in proportional fonts. Maybe because proportional fonts are easier to read so your brain subconsciously ignores them. And typos, like a misplaced or forgotten comma, can cause some of the most annoying bugs. Also, most editors still mostly operate on individual characters. With a fixed width font I can immediately see how many cursor up and cursor left commands I need to send to move the cursor to a specific position in the text.