This war was over around about the time I started, but my take on it is that it's all a bit intermixed and Mercurial lost because bitbucket caved and did git hosting [2], but git won beacuse of github and heroku [1][2]
Bitbucket and GitHub started around the same time. GitHub executed better. And the Ruby community adopted GitHub early. Comparisons around 2010 said GitHub's UX and network effects were top reasons to choose Git. Mercurial's UX and much better Windows support were top reasons to choose Mercurial.
And, I don't know if GitHub succeeded because of Git or the other way around.