Geographically once you get outside of major tech hubs, the Mac's market share as a developer workstation is quite tiny. I remember one time I was sent out on site to a big corp in an office park in New Jersey and everyone in the cubicles around me came up and cracked jokes about my MacBook, tried to challenge me because I wasn't a "real developer" since I used a Mac etc.

Honestly I'd guess the main people keeping the Mac line afloat at Apple are probably their own engineering staff. Imagine having to develop Apple products on another OS, how frustrating that would be