Even though Git is decentralised, people like having a simple client-server model for version control. So with Github being the most funded free Git hosting service it grew to being the biggest. They also built out the extra services on top of git hosting, the issue tracker, CI/CD, discussion board, integrated wiki, github-pages, etc.

I would say all of those things were present before the acquisition, enough that Microsoft itself started to use the site for its own open source code hosting.