Github sure contributed to the popularity, but I remeber distinctly as Git came out and how it took off like rocket. Git was a "killer app" from it's day of inception and everyone I knew switched their source control to it in late 2005 early 2006. It was a game changer to say the least. Github jumped on a already rolling bandwagon and left me ans many people I knew wondering why the hell you would need to host your projects there. (I am still a little bit puzzled but came to accept it as useful)
> Github fixes the problem that most users have with git (but are ashamed / too ignorant to admit): That it is de-centralized.
Git is designed for an environment where there are multiple canonical trunks. RedHats kernel is equally a master as SuSe's. So you are maintaining various tips in a semi-synchronized manner. In most projects there is a single repository branch that is the true branch (with perhaps a few tags for LTR) that represents the project. For that reason a lot of Git's mechanisms are unneeded complexity.
The killer features of Git is GitHub, and to a lesser degree local commits (after all, Mercurial has that too).
Anecdotally, I started using git because of projects on Github I wanted to contribute to. A number of others I know where in a similar boat. Before that, we used subversion, bazaar or mercurial. I personally am happy with having been pushed to using git and if it was winning anyway (not clear) I'm sure I would have eventually ended there anyway, but GitHub is the reason I started using it when I did.
It's tricky to compare them now, because Git won, it's got a lot more investment. Ideally you'd need to care them just before Git got the upper hand - but that's hard to pinpoint.
