(I am paraphrasing) Lucas said, "In the early 90's, I was faced with doing the Prequels or just going a totally different way for the movies I was going to make for the rest of my career. On one hand, I knew if I made the prequels, I would make enough money so I could be able to do whatever I wanted in the future."
In the early 90's, Lucas was still recovering from his divorce from Marcia, which he took a big hit financially, and Lucasfilm wasn't pretty much running even every year. Lucas figured, though it wasn't a sure bet, that another trilogy would make muchoo dollars, and he would finally be set for life, to make and afford any film he wanted to do.
Maybe that is the reason the first two prequels were geared to a younger (or childish) audience to make the most of the movie market? Lucas maybe was afraid to make an edgier movie like ESB, that it wouldn't maximize profits, and though TPM was mostly panned by critics, and so was AOTC, they both still made 900 & 650 million dollars worldwide, and that isn't counting toys, DVDs, etc. By the time he got to ROTS, though it was already a darker story, he was able to get a PG-13 rating, because the money was already made.
Just a thought? What do you guys think?