Because the Firefox devs think their browser should pander to the tablet-interface loving users, with advanced features hidden and the GUI dumbed down - while a large part of their user base specifically wants an "advanced" browser with lots of addons which does NOT look like Chrome.
I was rather shocked when I first ran Firefox on Android, I was expecting,well, Firefox, but what I got was some massively dumbed-down piece of junk that was worse than a range of no-name Android-only browsers from vendors I'd never heard of before. I eventually went with one of them, and at one point submitted a bug report. Within a few hours I had a reply, and a fix. It was everything Firefox should be, but isn't.
I've been a Firefox user since Phoenix 0.3, but none of my mobile devices runs it, and desktop is only hanging on because of all the plugins (of which, admittedly, about half are installed just to undo all the crap they've done to the browser in the last few years).
Still, at least they're hard at work on Firefox OS, which is what the market has been crying out for.