I mean, I don't think anyone can say for sure if "GitHub became GitHub because git was the winner" or "Git became mainstream because GitHub won the developer mindshare", pretty much everyone I knew used GitHub for everything besides the actual VCS protocol, although a lot of us early users were users of GitHub especially because of git.

Most people just wanted to collaborate on the platform other people were on, and where the popular projects were, that it used git was just an implementation detail at that point for most I think.