Git owes a lot of its popularity to github. Without it, there's a good chance that mercurial would have taken over. In addition, the centralized workflow was what made both git and github popular. It simplified git usage enough to let a lot of novices get started.

I'm in no way a fan of centralization that github represents. But I think a decentralized workflow using git was a lost opportunity. People complain a lot about the git-email workflow. But I see no reason why it couldn't have become as easy as using github if the effort spent on github was spent on git-email tools and user experience.