GitHub is bought by Microsoft. Mass migration to GitLab and other repository hostings have started. It’s an opportunity.
As already explained, pijul doesn’t work like git, from the interface to the most low-level internals. It’s not like we can can drop it in a service that’s made for git and expect it to work. Note that neither github nor gitlab support mercurial, though it’s quite used too: as always, it’s a matter of costs. It’s not trivial to adapt a tool that’s completely built around how another tool works to another very different tool — and pijul is much much more different to git than mercurial. People are migrating from github to gitlab; not from git to something else; changing host is a bother, and nothing more, changing the VCS is a totally different thing.