How can an open free Q&A site be concentrated on something? It simply reflects what people want and need.
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.
Compared to whole languages communities, for languages like PHP, Java, etc (and .NET), the number of "people who follow Spolsky and started using SO because of that" is miniscule. A statistical noise at best.
Very easily. For instance, if there was one toolset that either had worse user-facing documentation or a higher proportion of users who were bad at extracting solutions to specific challenges from general purpose documentation (perhaps because of mandatory use at the kind of places that employ such users), then one would expect that an open, free Q&A site with no deliberate focus would disproportionately end up serving that toolset.
There's a multitude of other ways such a Q&A site could end up with a skewed pool compared to overall use, too, that's just one example.