So many tools are tightly coupled to git or just assume that everybody uses git. I think that it's hard for any new VCS to gain traction without good git compatibility.

For instance Pijul might very well be a lot better than git / jj. I wouldn't know, I haven't bothered trying it because all the projects I need to work in use git. But since jj has great git compatibility, I actually have been able to adopt it because of its git backend.

A new VCS that doesn't have git compatibility at its core is going to have a really hard time overcoming network effects.