I shot him a message on Twitter asking why Brave was built on chromium, and he referenced this link where he answered previously:Efficiency. Same as everyone else.
Why do software developers develop for an operating system instead of writing their own operating system and packaging it in?
He may not have been feeling the love for Mozilla at the time after getting "resigned".
https://www.reddit.com/r/brave_browser/comments/9l0gnx/_/e74fcuo
From Brendan Eich (CEO):
We started w/ Gecko in 2015 at Brave, counted compatibility problems vs. switching to chromium, made the switch. Business decision, we agreed. Stop arguing for us to impair our growth with compat bugs that our users cannot fix.
No real choice on mobile. On desktop we started in 2015 w/ Gecko, did full evaluation against chromium, latter won. No benefit in going back to Gecko (Firefox faces diff tradeoff but on back foot now breaking XUL addons). WebKit lineage + Chrome market power => de facto standard.