The APIs are similar but the hardware requires a different appraoch.
For example touch screen input is very different to mouse input - you need to decifer imprecise user input... and then provide precise input to webpages that are designed assuming the user has a mouse. There are touch APIs on the web, but developers tend not to use those APIs because dealing with imprecise input sucks. For example press a link with your thumb, it will highlight. Lift your thumb, it will go to the link. But if you press, then move your thumb, then release... instead of clicking the link it scrolls the page. Unless you move only a little bit - then it does click...
And the only way to get "all day" battery life out of a 10Wh battery is by keeping the CPU powered off most of the day. Figuring out how to maintain the current state of the webpage, so it can be restored if the CPU is powered off and back on again, without breaking things like JavaScript timers/etc.
FireFox has solved those issues (and others) on Android. But while Android has similar hardware, that operating system is nothing like iOS.
All the work to get Gecko working on Android made sense back int he day, when Android didn't have a good rendering engine. It would have also made sense back in the early days of the iPhone when WebKit was nowhere near as good as it is now. But today, when someone else has already figured out solutions to every problem? Is it worth reinventing all those wheels?