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".