Theory:
Based on all of the information gleaned so far, we already know that Nero went back in time (as explained in the "Countdown" comic) and wiped out the USS Kelvin, an event which altered the timeline. Not drastically, but enough. When Starfleet finds out about the Kelvin, they assume that they're facing a Really Big Bad (I'm pretty confident that Nero lays low for now) and go on a technology upgrade tear to fact this new threat. Hence, the alternate USS Enterprise looking a lot more technically sophisticated than the prime USS Enterprise.
(As I said, it's only a theory. I'm just glad to see that we'll be getting some good Trek to take that horrid Nemesis taste out of our mouths...)