Because brave has a questionable business model where they, on one hand, block third party ads and trackers, and on the other inject their own ads, tied to their BAT cryptocurrency (the latter part being opt-in IIRC).

They also:

- added their own affiliate codes when visiting some sites [1]

- installed VPNs without consent [2]

which also taints their privacy focused posture.

[1] https://www.theverge.com/2020/6/8/21283769/brave-browser-aff...

[2] https://www.androidpolice.com/brave-browser-windows-vpn-with...

According to Mozilla's "Firefox vs Brave" page, the foremost reason is Brave's ad blocking might be too good and block pages you don't want to block.

The ones that give Google money to Mozilla, presumably - but there are other problems with Brave.