Mercurial had a better interface and early on was equal or better (as far as I know) for most things. Everyone went with Git because Linus made it. I think it also might have been faster for huge code bases... but that pretty much only affects the kernel team and a handful of others. Your standard CRUD app could easily use a slower VCS without noticing.
Instead of going on merits, everyone just followed Linus. Which is what always happens in technology communities. Everyone just does whatever some guy at the front is doing.