Probably because Linus Torvalds wrote it, and also because GitHub was actually quite a significant improvement over SourceForge. I know some people "love to hate" GitHub, but they made some really good "developer-first" UX improvements that many copied for good reasons. And lastly, Linus Torvalds wrote it.

I still maintain that Mercurial is far better for most uses. Every time I mention this I get a flurry of technical replies about stuff most people don't even know about (HN is strongly biased like that, which is not a bad thing, just something to be aware of), but most people just want to commit, push, pull, and some related stuff like that. Git makes a lot of simple common stuff arcane to make the complex stuff easier, whereas mercurial makes the simple common stuff easy and keeps the complex stuff arcane. This strikes me as a much better design philosophy.

Git and Mercurial are a tool designed to facilitate writing code (which is also a tool designed to solve problems). If you need to spend significant time to "get your head around it" then IMHO something, somewhere, has gone wrong.

I still use git because it's just the pragmatical thing to do at this point in time, and to be fair there have been active efforts by the git team to make the common/simple stuff easier in recent times, but yeah, I'm not a fan.