Because Chrome will instantly start to out compete it. And once Safari is pushed down below 25% share on iOS then sites will feel more and more free to avoid testing on Safari and push users towards just using Chrome. Not having two different engines to have to test against means that even more the web becomes something that works on Chromium by default and is totally broken on Gecko. That makes FF even worse, and Google will totally control Chromium and the engine that browsers like Edge and Brave work on top of and will be able to dictate (in the "dictator" sense of that word) what the engine does (and while they might be somewhat responsive to Microsoft, they won't be to Brave). And we already won the battle to keep Windows from tying the browser to the O/S on windows and watched while we lost the war and Google completely took the engine over. Cracking open iOS is going to let Google take over that platform as well, it isn't going to make life any better for Gecko or any other alternative engine.