Mostly because the browsing experience IMO is much much worse with Firefox. I tried extensions to get functionality back, it made it worse - slower, buggy, extensions would stop being developed etc. I wish Firefox was better, I really do. But IME it's frozen functionality like it's 2010 or so. Like, they have tabs, who hoo. I really find save/restore, multi window control, tab stacks, sessions, workspaces, and easy UI config pretty important in day to day use. That said, I also think ads are a deal breaker, but I really wonder if this won't bring back some of the ad-blocking proxies you run locally or something.
Or, someone forks chromium to keep Manifest v2 or whatever.