Isn't one of the biggest knocks on the whole android platform the fragmentation? Different manufacturers doing things differently, different size devices, different size screens, and different versions of the OS installed on each?
Apple has residual fragmentation caused by the evolution of the devices (or politics, if you're of that persuasion)... adding a whole new device "just because" would add to the fragmentation of iOS, frustration of the user base (why can't I run Siri, Turn-by-Turn navication, etc)...
I just don't see any real, compelling reason for Apple to go there.