the fact that almost by definition stuff that jj does is possible in git makes it hard for some folks to let go of the baggage that git has; it's simply hard to imagine a world where you can't git add ('how do you commit what you need committed and not commit the rest?') or not having to resolve conflicts immediately ('why would I want not to?')

...and it turns out when you answer these questions differently ('working tree is a commit', 'conflicts can committed) but still want git compatibility, jj kinda falls out of the design space by necessity.