InfoDroid really summed it up well. I think Lucas is afraid that people would find the original films better than the "improved" ones. With all the rumors about future changes, maybe he doesn't want to release the originals at least until he considers the movies finished. As a marketer, his worst fear has got to be realeasing the originals would make a lot of people completely satisfied and feel no need for his newest version. Still, he could make a lot more money releasing the originals, even if they were only available with the special editions.