A lot of the development ecosystem had integrations with Github, and haven't yet done so with Gitlab. Additionally, network effects matter, especially when if you add collaborators to your repo it's almost a given that they have a Github account.
I assume you're talking about CI? Not much more than that. But that only works in the most trivial cases. One repository, simple tests. I spent tons of time over the years always tweaking this and that to make whatever CI system we were using somehow work with GitHub. Lot's of setup, lots of trouble.
Not just CI. Various bug trackers, project management tools, static analysis tools, etc.