Then he got greedy. Hey, that's the real reason he made the Prequels. Greed, man!
I'm sure he signs a lot of deals thinking about the money first (as would we all, if we're being honest) but when you hear him talk about the prequels, it seems as though he genuinely wanted to do something ambitious.
Furthermore, it sounds like, if he had made Episodes IV-VI with today's technology, they would have looked a lot like Ep. I-III. He was always ambitious, but the technology and budget constraints (at least of the first movie) used to keep him in check; kept the original movies feeling gritty and real, focused on character and plot development instead of special effects.
The worst thing to happen to George was that his dreams came true. Suddenly he could make his vision hit the big screen exactly how he imagined, and it turns out his visions are a little too... "grandiose" for many of his fans' tastes.