The reason why, is because, much like each of the three Lord of the Rings films, expect an Extended Edition (spread over two discs) to arrive on DVD and blu-ray sometime in the fall. There's been no official announcement, but if I'm wrong come Christmas....oops.
The Extended Editions for Lord of the Rings does have more footage put back (seamlessly) into the movie - about 30-35 minutes for Fellowship of the Ring, about 40-45 minutes for The Two Towers, and about 50 minutes for Return of the King. Rather than just having deleted scenes put back in (like many director's cuts, or unrated versions, etc..) this footage actually added to the films. Sure, it's easy to see why some scenes were taken out or shortened for the theatrical run, but these scenes were filmed for a reason. It just adds more to the world. More story. Given the success of those editions of the Rings films, expect the same for The Hobbit prequel trilogy. Just a head's up if you don't like double-dipping, or waiting for a better, more elaborate version of something you know will (most likely) be coming out down the line. Don't worry, I'll keep you posted, as those are the versions I'm getting myself.