Github became popular because people wanted free Git hosting, not the other way around. Most other Github features just get in the way.
Also, instant branching are really convenient feature. I remember the time when I was using Darcs that I got lost pretty quickly with 10 copies in 10 directories.
I disagree wrt Github. A lot of projects I was involved in wound up switching from git to mercurial because it was too difficult to manage git master repos on a server compared to Mercurial. Github solved this neatly, and the issue system has allowed my last two companies to switch from Mercurial+FogBugz|Trac to Github. That's useful consolidation and lower admin overhead.
I think there was a co-evolution of the two. A lot of people started using git because Github was where the cool projects were being hosted. At the time, I recall hoping for bzr to "win". I like git these days but coming from an svn is king world - bzr was a bit gentler for transition.
* Linus was using it for the kernel, so it became mature slightly faster (because it had more people's attention.)
* Github happened at the same time Sourceforge stopped being cool (this is the same time that Digg and Reddit started beating Slashdot.)