As a web developer, my job is already to chase down spec-violating implementation bugs in Safari, Chrome, and Edge. Having one more user agent out there does me no good; I'm disinterested in increasing my pain to support a 4% userbase.
I'd rather browsing be the purview of one or two giant, well-regulated players than a thousand minorly-incompatible little flowers.
Key: well-regulated. These giants are anything but. And in the past several years Chrome basically said "I couldn't give two craps about Safari's or Firefox's opinion, we're going to ship our own APIs and call them web standards".
Also, one of the biggest problems with Chrome has been that Google are breaking standards with it repetitively. If you're for well-regulated browsers, Chrome shouldn't be your browser of choice.
It's okay; FF unbroke themselves in 2 revisions anyway. But if it'd been Chrome, fixing the site would have been our top priority.
But if there's a bug where either a browser deviates from the standard or different browsers implement an ambiguity in the standard differently? Web developers will fix that bug in Chrome, Safari, and Edge before fixing it in Firefox. For almost every site, that's what the cost benefit analysis looks like.
If my site doesn't work right on Chrome or Safari, that's my problem. If it doesn't work right on Firefox, that's Mozilla's problem.