I was using git before github was even a thing. Ruby on Rails community was already moving to git in 2006 & 2007. Github was created by a couple of Rails developers that saw the popularity of git and responded.

I also tried Darcs, Bazaar & Mercurial back then. Those options were slooowww. Git largely won because it was blazing fast, and easy to understand. Probably having the cachet of Linus helped. I personally found transitioning from svn to git was a breeze.

I do have some quibbles about the structure of commands & arguments from an UX perspective with git, but ultimately it wasn't a dealbreaker.