Git had git-svn. I used it for a while to interface with a project that was still on subversion. So, I was able to gradually get my team off subversion. Subversion in turn had an easy migration from CVS and of course CVS was a bit painful to use so lots of people ended up migrating. I did such a migration once. It took a while to run but we got it done and kept our version history. We did not look back after that.
There probably is already some work on this but the path to success for pijul would be removing as many obstacles as possible between git and pijul and alternative systems such as mercurial. Make migrations easy. Make interfacing with git remotes easy for pijul remotes or interfacing with pijul users easy for git users.
Git is well entrenched of course with web UIs, IDE integrations, CI/CD support, etc. So there's more to replacing it than just writing a bunch of cli tools.
Sourceforge was pretty big at the time. And they lacked git support for quite some time. Github basically burried them by the time they figured out they had a problem. Of course it still exists but it's not a very obvious place for people to park their code at this point.
Those systems had undeniable pain points, and so does Git.