Your wording gives the impression that you are implying that Firefox is different. clarifying for anyone who doesn't know: Firefox on iOS is just another Safari wrapper, just like the others.
I think the simplest explanation is that people like to stick to what they use on desktop. Firefox has a 6% market share there so it makes sense it would also be lower on mobile.
Amazon and Apple both picked their names to show up early on in lists, and anyone who remembers the phone book days remembers all sorts of plumbers with names like "AAA plumbing"
The British company Acorn Computers picked their name to appear even earlier in the alphabet. Acer of Taiwan beat that, is there any company earlier than Acer?
(Acorn was the original "A" in ARM, the Acorn Risc Machine.)
why is firefox ranked so low in popularity to other essentially wrappers of safari?
marketing perhaps?