Its because he goes back and changes it so much. Star Wars was a lighthearted adventure film, nothing more. ESB turned the series into a deep, mythological fantasy film for adults. ROTJ turned it into a poignant tale of redemption. And in each film he used as an opportunity to indulge in one of his fantasys, eccintricities and tastes (i.e. interconnected epic, primitives versus technology, pirate battles, artificial characters, visual tone poems). The Lucas that made ROTJ was not the same Lucas that made ANH. And in TPM he was a completely different person, so the PT is completely and utterly different in its style and tastes because Lucas was different. But because he was making all of these as one connected tale he was forced to assert and present them as continuous and consistent, even though the material and content was very inconsistent. Had he made them each as their own self-contained series or film this problem wouldn't exist, but he tried this in 1987 with Willow and it failed--so rather than develop new material he allowed himself to experiment and indulge his various sensibilities by selling the material as "Star Wars." To complicate matters, he had inadvertently created a modern mythology and a cultural phenomena, which he clearly did not intend nor want any part of (who would?). Fans demanded more and if he didn't then he would be publicly hounded (ie what happened from 1984-1992, as well as now with the OOT) and so he gave in--its just that he could not stay in the same bubble forever, so even if he said he was making films with the Star Wars title, inevitably he was a different man inside and thus the films were not the same, nor completely consistent--I don't even think he notices, let alone cares. Its too hard to be objective when you are so close to the material. Star Wars has, unfortunately for him, become his life, and he has been cursed with tunnel vision ever since 1980, as inevitably happens.