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.
UI and pricing aside (I don't have much direct experience of either on GitLab), GitHub is, AFAIK, by far the most popular and therefore it's easier to get your project discovered and get other developers to contribute.
I do kind of think that by centralising so much stuff on a website owned by Microsoft we are running the risk of another Reddit-like situation where GitHub turns sharply anti-user in an attempt to monetise in the future. But for the moment, the network effects are real and significant.
Drive by contributions are just as frequent in gitlab as github. I've been involved in moving multiple large free software projects from github to gitlab and there was zero difference seen in discoverability, in fact contributions grew on gitlab, although there is no data to suggest it was because of gitlab, as it could have been because projects aged and grew