My point is that their UI could change, and it's easier to test out UX on something that you'll use everyday, than on something that you'll never use. They made Github because they wanted to use it themselves, not because they saw it as a money making scheme.
For productivity apps (and especially developer tools), it's not worth chasing customers on a platform you don't love -- you'll do a bad job and you'll be unhappy doing it.
And as Mac users, they may be consciously avoiding the reverse of an all too familiar situation, where non-Mac-using Windows developers shoddily port their app to OSX.
