Lucas is greedy, simply put. You realize what he's going to do here of course - he's going to milk it for everything it's worth so he can get the maximum amount of profit from these sales.
First he releases the original Star Wars trilogy on VHS, then the THX edition, then the Special Edition, then Episode 1 on VHS, then Episode I on DVD, etc. etc. And now he's going to release Episode IV on DVD, then E5 and E6 separately, then all of them together, then the Speical Edition of all of them together, then just Episodes 1-3 in a box set, then all of them together, then Special Editions of the new ones, etc. etc. ad nauseum.
I like the movies, but Lucas is a greedy bastard. And the above is why I have never, and will never, buy an actual copy of a Star Wars movie.