Why GitHub?

I can't help but notice most (all I've seen anyway) of the federated projects are hosted on GitHub. GitLab is also not federated, but can be self hosted and has at least discussed it.

• Lemmy on GitHub
• Mastodon on GitHub
• PeerTube on GitHub
• Friendica on GitHub

I am fully aware of my bias for GitLab over GitHub, but I still wonder why is those things? Is there a federated source hosting project?