The only time I ever use a chromium based browser is because Firefox still doesn’t support WebSerial

I think its convenience/ignorance. The general population isn't as tech savvy so they'll use whatever is popular/well known.

Chrome is advertised as "the fastest browser" while edge comes with every single copy of Windows by default.

I keep chromium installed for the few times a year I visit a page that doesn't support Firefox.

I have to use chrome and edge for work. I was specifically told we aren't approved to install Firefox..

Chrome is convenient for getting into gmail but that's about it.

Edge is comfy. Simple as. I don't use chrome because it's too resource hungry (or was last time I used it), and I see no need to switch into firefox. Especially that I have, most probably hella outdated, bias against it from around the time chrome launched. Firefox was that messy weird browser nerds used, always full of tab and bars for some reason (ones that weren't the effect of a virus).

When Edge goes with manifest update, I am aiming at Floorp.