Because brave is open source and Vivaldi is not.

I do really like Vivaldi on mobile - customizes perfectly for my needs. Have no reason to mistrust them regarding their proprietary chromium gui.

Vivaldi user here. Excellent browser.

But I still really miss Opera from the old days. Vivaldi is the next best thing.

I used Vivaldi for a good year or two on my machines, but after some point Vivaldi was so slow to launch and to navigate. Maybe it was my setup, but other browsers were quick launch and use. Been using FF for several months now, maybe they fixed the issues in Vivaldi since.

Vivaldi has this issue with many tabs slowing down the whole UI, but the new v6 feature of workspaces allows you to move some tabs to a WS group, that improves UI performance

But yeah, that's one of their biggest issues

Vivaldi is not available on mobile (ios)

Vivaldi on android still does not support adding custom search engines.

Brave's blockers are better, the browser performs better, and I like the native Chromium tab groups more than Vivaldi's tab stacks. Brave also had background media playback on mobile long before Vivaldi, who only got it in the latest update.

Vivaldi's fine, and it's what I'd use if Brave didn't exist, but I just like the experience of using Brave more, especially on desktop. Vivaldi's new mobile version, from what I've seen, has clarified their old mobile UI a lot and got background media playback, so that's less of an issue.