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. ;)
I prefer Mercurial for mostly cosmetic reasons (I believe that Git and Mercurial are very close, feature-wise). I prefer having one command "hg" ("hd add", "hg serve", etc.) to the dozens of commands for git. I think it has more to do with the way that I think than anything else: Mercurial commands are easier to remember for me.
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.
