Lots of people were using svn and mercurial was also coming up around the time that GitHub launched. Both git and GitHub were superior to all the other options but for many people they did the switch to GitHub and git at the same time.
It's interesting to me that the only thing that made me vastly prefer using Github over bitbucket is that Github prioritised showing the readme over showing the source tree. Such a little thing, but it made all the difference.