Most of these chromium-based browsers are intended to address privacy concerns. Firefox (mostly) respects your privacy.

There are also sometimes compatibility issues with Firefox because web developers only test on chromium and webkit. Anyone opinionated enough to put up with that is just going to use Firefox.

>intended to address privacy concerns

Primarily probably yes, but I think for example Brave or Arc Browser teams also had ideas for their own browser features instead of "just" making a more degoogled Chromium. Helium as well, I suppose, otherwise what's the point?