Its because its good enough for him, which I guess is the problem. "Episode II and III were shot in 1080, you saying there's something wrong with that?" Whose going to argue otherwise?

I don't think they were done in that res to "match" the prequels, but there was no reason to scan them higher because the prequels were that res and Lucas thinks the prequels look great (also, because its digital, he probably thinks the movies will never be put on film again--and he may be right--so for a direct digital copy it works for him). To him, I think he doesn't want to spend the money when he doesn't think there's any reason.