I'm making an assumption that the majority (if not entirety) of GitHub employees use a Mac, so why wouldn't they make an editor for themselves on a platform that they use? I doubt they're doing it to bolster the OSX ecosystem as much as meet their own needs.
Because the product they provide is tailored to open source/free software. No matter how much you might be enamoured by Apple and its polished products it is hard to get around the fact that these are the opposite of open and free.
Remember what happened when Linus decided he wanted use BitKeeper to maintain Linux? Also remember how that worked out in the end? Free and/or open source software and closed, proprietary companies often make odd bedfellows.
