Because, given the way contracts are in the US, such a move doesn't in any way benefit the consumer in the US. That, of course, isn't the same overseas where it is prepaid.
But honestly, as a customer, I could care less about Apple innovating for its own profits only. It is only interesting from a business sense and from a shareholder view. Innovation to a consumer means, really, something new. The fingerprint censor is an innovation (but really a gimmicky one at this juncture that might mature into something useful). The faster chip, for most customers, is not. Similarly, a chip that allows Apple to build only one model? Worthless to most people. And, besides, those chips are generally built by Qualcomm, no? That's like saying Apple innovates over the Haswell chip.
Even though Apple still has to provide things to work well with the Haswell chip, get good life, not overheat, etc; but customers don't see that as innovation, as the prior model did all of those things fine.
Innovation = Bells and whistles and gimmicks, usually, to most people. Apple doesn't do that much until it releases a new product. They evolve from there. Hence why Apple is attacked for not innovating. An innovative iPhone would be an iPhone with a larger screen and user allowable code, simply because that'd be new for the line. That doesn't mean it is a good innovation (if you don't like the new screen/new software capbilities) or that it is actually innovative (jailbreaks aren't new, and big screens exist). That's simply what a customer thinks is "innovation"
Or you can call innovation "differentiation from existing product lines"
