Even if Pijul is better (not voicing an opinion here), it's not drastically better enough to replace Git in widespread use in the near term. The difference in day to day use is not huge.
And it is better in handling certain merge situations that are painful in git, but most programmers don't run into these often enough to care.
> What makes this unique?
It's patch based rather than snapshot based.
> Why should people care?
It avoids certain merge scenario problems that git makes painful.
> when I started using Git, interfacing with existing svn repositories was a key selling point for me
Pijul can import and export to Git (and probably others) with less problems than git vs. svn (because SVN is not distributed and had a weird branching model).
> Show that there's an ecosystem
Here's the chicken and egg problem again.
There's a "free" hosting service advertised on the front page. Or you can use it with your git hosting (but lose some of the advantages).
But GitHub and GitLab have their own CI systems and other infrastructure which isn't going to be easy or cheap to replace.
So yeah, I think that Pijul is a great piece of technology that solves a real problem we have with Git, but it's unlikely to overcome the inertia that Git{Lab,Hub,} have.