git before github existed was a pain to use. I honestly just continued to use SVN because the tool support (like automatic renaming in IntelliJ when refactoring a class) just worked, and there was concern about exactly how to back up our source code.
After github, things became much easier and it is likely the main reason people use git over mercurial.
Of course, now I work at a place that uses Mercurial, because Git can't handle the size of our repo (an order of magnitude, at least, bigger than the Linux kernel)...
git even before github was a game changer for distributed development, kernel being the most prominent example. It did not require random people to have connectivity or authorization on some central SVN server, and yet they could clone and sync their trees and create branches to their heart's content.
But mercurial was also in play then and seemed better (which is why I chose it) or at least on par. Somehow git got more traction and is now the default.
After github, things became much easier and it is likely the main reason people use git over mercurial.
Of course, now I work at a place that uses Mercurial, because Git can't handle the size of our repo (an order of magnitude, at least, bigger than the Linux kernel)...