Jon Skeet is a Google developer. Well early Git adopters were Linux developers. Does it mean that GitHub is not really mainstream apart from a narrow group of people?
Does git even work well on Windows yet?
So, if git didn't work fine on windows for a long time, there would be plenty of time for network effects to entrench github as a non-windows site, and indeed that is the case. Github is where you'll find a lot of javascript, trendy open source projects, and even os x apps, but not so much windows stuff.