Because it is still Chromium based and it means it is fast on Android, plus it comes packed with an adblocker by default which works wonders in closed out systems like iOS, also as many browsers (not all of them) it supports account syncing which it is always a nice plus (I can use a good working version of Brave in all the systems and keep a good flow for example).

I main Firefox in pretty much all the systems, but the Android app is missing a lot of features like tab management, and the iOS client just sucks (Brave works better there despite being Safari based too).