If I had to guess, this is them meeting other Open Source contributors where they usually are, which in large part is GitHub these days.

Out of 28 projects whose release note RSS-feed I subscribe to, 25 of them are hosted on GitHub. While I'd love to see more of these projects move away from GitHub, it is understandable that they go where the largest amount of devs are. I'd love to see more of them start mirroring their repositories to Codeberg or their own Forgejo instance though, to give developers the opportunity to contribute while not alienating the devs who stay on GitHub. At least that would lessen the loss of opportunities for the devs when ditching GitHub - but I am not sure whether it is trivial or a hassle to maintain that kind of setup.