Because it’s so damn easy. I started contributing to OSS and creating repos on GitHub when I was 16. I was not able (or interested in) managing my own git server; I didn’t have any connections to Apache.org. Sure I could’ve emailed diffs to some mailing list, as I know many people have done for years, but GH is a vastly better experience.

Github was so accessible that it made possible what otherwise would not have been.