That is an interesting observation. Perhaps it is due to habit. After 45 years of programming (started with punched cards), I am accustomed to fixed-pitch fonts. Proportional fonts for code seem unintelligible to me.

It is worth noting that all quality programming publications display code in fixed pitch. Knuth, in his epochal "Art of Computer Programming", uses fixed-pitch serif for source code. Petzold's "Programming Windows" uses fixed-pitch, as does Kernighan & Ritchie's "The C Programming Language".

Because I work with data pretty often, and working with data pretty much requires a monospace font, and there's no reason for me to switch back and forth.

For example, cli mysql wouldn't be too much fun in Geneva. There's a nice visual hint between labels and data in lines of whois output, and that hint would be icky in a proportional typeface:

    status:      reallocated
    owner:       NETLINE PERU
    ownerid:     PE-NEPE4-LACNIC

Also, log files with columns like timestamp [ip] etc., and some particularly friendly configuration files.

And, although it's not very common, I occasionally use it as a typo-checker. If I have a function or a class where I can't easily abstract out some kind of repetition, then using a monospaced font with variables that are the same number of characters lets me visually see right away if I've mistyped something. I couldn't quickly find an example, but it was the first programming-specific monospace font thing that came to mind.

I actually tried a programming font that is not monospace and it's not an issue. Left side is aligned the same anyway and I don't care if lines don't align on the right side.

Alignment probably more than anything, though obviously the necessity for that depends on the language and style.