Because they themselves don't deserve it. I need a browser company to focus on making a good application, not focus on telling me what I should see on the web. Mozilla's definitely interested in the latter, but I don't remember the last time they actually introduced something that made me go "huh, that's good". The company are activists hostile to me and mine, seem to build a worse and worse product in and of itself (discounting devs not building for Firefox) and are funded by Google. What's there to like?
The privacy disaster that's new Edge is constantly doing really good UI innovation with powerful but simple features like collapsible sidebars while retaining native UI.
Vivaldi, well, the whole point of the browser is features, and they deliver. They have three different ways to use tab stacks, for heaven's sake. And they do privacy, and aren't interested in telling me what I should see.
Brave, same. Privacy focus, try to build standalone revenue streams. They actually ship new things, if less aggressively in the UI department than Edge or Vivaldi. And they're not interested in telling me what I should see on the web.
I'd gladly use Mozilla, but they have to deserve it first by being genuinely good in terms of attitude and/or product, preferably both, not just being "not Google". By my count, product's getting worse and the attitude is complete garbage.
This rant is also more or less on point: http://dpldocs.info/this-week-in-d/Blog.Posted_2021_09_06.ht...
Google, Microsoft and Apple all push their browsers forward by their external footprint - Google and Microsoft cloud platforms, and all three by being the default.
Brave's building their own platform, Vivaldi a bit of the same, but also just built-in tools in the browser like a simple notetaker and mail/calendar/RSS apps.
Firefox has Pocket, which is good and a good idea, but their userbase is stupidly hostile to ventures like that, the only thing they accept is a pristine FOSS project funded by donations.