Cheaper hardware, Steam (and other games in general), testing on the platform used by a majority of our users, and more customization available than OS X, for starters.
“testing on the platform used by a majority of our users”
This.
I think that's also why patio11 doesn't use a Mac, If If Guess Correctly.
dat troll
purposefully tried to say it in a way that wouldn't seem like I was trolling. Alas, this comment came up anyway.