There isn't a logical one other than Apple determined on their own that nobody needed one. Doesn't matter if the average consumer felt otherwise. In all seriousness, Apple would rather have the consumer pay $100 more for twice the memory capacity (16 Gb vs 32 Gb).
I just saw this video and now I have to ask the question. It really makes no sense at all if you think about it, I mean this is suppose to be the best phone in the world...right!?