Git won over Mercurial simply due to Github. There were some other minor contributing factors - association with Linus, speed - but they are insignificant compared to how popular Gihub was (for good reason) and therefore how many people were exposed to git.
The Mercurial alternatives like bitbucket just didn't have the same spread, and we got stuck with year after year of teaching new people a difficult interface.
Git was more popular for C, Perl, and Ruby. Mercurial was more popular for Java and Python. It was far ahead on Windows. Google and Atlassian bet on Mercurial well after GitHub existed. The idea that only one could win would have been strange to a lot of people at the time.
I still push for SVN when we're doing work that centralization makes a great deal of sense. For example, I look at SVN+puppet to be an exceptional combination... and I really don't need that puppet repo to be distributed.
Then again, if I'm working with people in different areas, and want them to have a full reproducible copy of the repo, git it is.
I maintain a "use the right tool for the right job". Sometimes the Cathedral wins out, and other times the Bazaar wins out (NO! not the Bazaar source control!).
The Mercurial alternatives like bitbucket just didn't have the same spread, and we got stuck with year after year of teaching new people a difficult interface.