First, git is decentralized by default. GitHub has very little secret sauce here; most, if not all, of their cool git tricks are actually pushed upstream into the git client to benefit everybody. For remote management, I can run `git init --bare` in any directory on any ol' ssh box I want and then set it up as a remote and get the same exact `git push` experience. Or use any of the strategies that git supports for interacting with remotes.

So GitHub chose a great decentralized tool as their foundation, and then got the community UX right. There's easy permissions management, easy SSH key management and instructions, issue and PR tracking, easily-accessed discussion, etc. GitHub is 95% about community and 5% about easy repo management, IMO.

That said, they're no longer the only player in town. They're the most universally understood, but you can easily set up a GitLab CE server on your own hardware if you want the same thing for free, but there are management costs associated with that.

I think the other critical reason GitHub is associated with open source software is that, after all, it's 100% free for open source software. You get all the above features for free if you don't discriminate about who can see and clone your code. GitLab will give you free private repos, but in my opinion I think GitHub takes the better stance here for the Open Source world. "You want free tools? You have to either pay up or share."