I don't think GitHub is popular because it's just a web front-end over a DVCS.
I'm basically a mercurial user. Use it for work. Used it for all my personal projects. The commands and workflow seem much cleaner and make much more sense to me than git's. I switched to git for personal projects soley because of GitHub.
Partly because it has a very good user experience and is snappy (bitbucket is slow in comparison) but mostly because that's where everyone else is.
I'd go as far as to say that GitHub has done more for git than git has done for GitHub.