That comes back to the point I made earlier, that at some point Lucas became more concerned with the Star Wars franchise as a profit-making machine rather than a creative endeavor; the filmmaker got replaced by the brand manager. And since he was the writer, director, producer, and owner there wasn't anyone who could step in and say "Hey, we should focus on making a really good movie first."
You can even see this a little bit in Return of the Jedi, where the original plan to have wookies got scrapped in favor of kid-friendly, more-marketable ewoks.