I'd guess that this is more due to the popularity of GitHub among potential contributiors to these projects. I've seen it mentioned many times here on HN that a potential contributor may not want to put in the effort to create an account on another VCS provider or send a patch to a mailing list, whereas submitting a pull request on GitHub is a much less involved process for them, given that they likely already have an account and are familiar with the process.

However you could definitely argue that given that the projects in question are alternatives to GitHub, a developer interested in contributing will likely be invested enough to sign up for the alternative service.