Firefox OS never made sense unless Mozilla was prepared to pay the backroom bribes and incentives to get it preinstalled on hundreds of millions of phones. And they weren't.
I think the main bottleneck is not the installation by the vendors, but the apps available.
I believe, what is missing is an abstraction layer between the apps and the OS. If it was possible to create apps for Android and Apple in a generic way and, it was easy to add a third OS, so all the apps could be made available to the new system, a new OS would be not so limited. I know, I know, that is itself a very difficult endeavor.
The thing holding them back is apples deliberate crippling of them to make them hard to install, slow, buggy, and not having basic platform features.
I believe Apple is deliberately doing that because if they supported PWA's properly, they'd loose all vendor lock in.
imo pwa's on webkit are only crippled in regards to the google extensions that make no sense for most usecases but ad-tracking.
Classic example: Oil companies "investing" in clean tech in the 90's, sucking up government grants and public attention and never really getting anywhere probably slowed down the switch to renewables by 10+ years.
This is exactly my take on it.
I think timing was a big part of it, too. They were too late to get a foothold on the current market, but too early to take advantage of the more open platforms coming out nowadays like the PinePhone.
Look at the Chinese Manufacturers having trouble to deploy Android due to tech embargos.
If those could just install a free os made by an indendent Organisation...
It would not help to overcome embargoes if said independent organization is established in California.
The only issue with Android is that it isn't really opensource - users expect the Google apps, and those are closed source.