Maybe that's it? The product category of "smartphone OS" is now so well-defined that the possible improvements are evolutionary at best, not revolutionary, and hence there's also no space for a new entrant to provide a big enough improvement over the incumbents to gain significant attention?

Firefox OS promised to make app creation easier through web technologies, but "not enough apps" isn't the problem with Android or iOS, quite the contrary, so for an end user, that's pretty irrelevant.