For me it’s because Firefox is (or at least was) noticeably slower. Didn’t support all the extensions I use. And didn’t allow YouTube playback with audio beyond 4x play speed.

All of those items led to me to choose brave over Firefox since I encountered every one of them on a daily basis.

Also I hated the default font (or perhaps it was some other quiirk of the layout) of Firefox. I couldn’t figure out how to fix it.