Hm, thinking about it.

Some of my attraction to github is just that I am used to and like the UI, and I get annoyed on another platform where I don't understand the UI and don't know where to find things or how to do things (like set up free CI for open source project, or debug existing CI for a PR).

Other than that... auto-linking to issues, prs, and code (with auto-expansion of excerpt in issue) cross-project is nice?

But you are right, git is theoretically something where separate projects are fairly isolated, which does make it more mysterious that competitors are having trouble and it seems like github has some kind of network effect lock-in anyway. It's a good question.