Because the alternative is that Blink becomes the new IE6.
"Moral cause" = many people will walk right away from even expensive software over these kinds of things.
Just saying.
I started using Firefox in the late 2000s mostly because I switched to Linux, and Chrome wasn't a thing yet. The web dev tools were better (firebug, anyone?), there were extensions like pentydactyl and whichever predecessor it had back in the day, and overall I just preferred the UI.
Pretty much every reason I originally used Firefox is gone now. I used it out of habit now and because it's less memory intensive still, but that's it. There's just not much going for Firefox except being backed by "the good guys" in a fight against the Empire. Even the cross platform support has withered with all the Chrome components like Skia and dependencies on tools like node.js (ironic that Firefox needs chrome's javascript engine to compile) limiting it to mainstream target triplets.
Firefox is trying to stay usable and I applaud that, but now they're just perpetually stuck trying to play catch up with Chrome's features and performance, always lagging behind, either a little or a lot. Chrome sets the web standards -- I think Firefox has simply lost.
People sticking to their principles and willing to use a less advanced browser just to guarantee its improvement is what gave us the fast browsers we have today.