As a Mercurial fanboy, git took off thanks to being from Linus, and a requirement to contribute to the Linux kernel, from there the adoption wind was in motion.
As someone who was a mercurial fanboy too, critical element was that Github happpened for git while mercurial had nothing comparable - and before anyone points at bitbucket, it fucking sucked in UX
It was huge problem for Git adoption, that's why I mentioned GitHub
EDIT: GitHub had probably the first UX where I actually liked what I got, compared to various earlier git based ones, or the horrible CVS and SVN ones where clicking on a file name definitely didn't do what you expected.
I barely remember Bitbucket from the mercurial era but I do recall that while it was better than some, it was still worse than GitHub, whose "here's default branch's HEAD, plus auto-opened and formatted README" was really a game changer
