I'm not an expert in git (still learning) and I've hardly used Mercurial at all, so right now there's no particular reason. There are probably better people to ask that question. :) I'll try hg in a project sooner or later, but learning to get to grips with 2 version control systems at once while trying to do something productive seemed a bit much. There wasn't really anything on the feature lists that tipped the scales; the Linus Torvalds association and the resulting curiosity probably did.

I can't believe I fell for a celebrity endorsement. ;)

Why git? It's popular, which means it's easy to find people to ask questions when you need to.

Really, I'd be fine with any fast distributed VCS, which means nearly all of them these days. (sorry Darcs). The ease of setup ("If you have SSH you have a git server") feature is also very nice, but I don't think it's exclusive to git.