Reasons: I haven't found a proportional code font I like very much and, more importantly, many—MANY—existing code files include text arranged with the expectation of monospace fonts and if you use proportional, they become a mess.
All respect to those who like proportional fonts, to each their own, but I think of code as a 2d grid.
Same reason I use syntax highlighting: color is another dimension that helps me see what I’m doing. This is particularly notable in Lisps, where turning on rainbow parens can be like switching on a light switch, immediately helping me understand the shape of the forms.
