Too much control.
George's work was amazing in the Prequels because he did his part like he should have and he let everyone do their jobs, not to mention that some people were hierarchically superior to him at the time.
In the prequels, he had too much control, it got out of hand. People who could have helped him flesh out his visions and ideas in a different (and perhaps better) way did nothing, some out of fear, others out of failure to be heard by George.
The guy may be not that bad, but he had become very self centred over the years. He barely listens to the pleas of the fans, no matter what side they may take. Georges just does his thing for his own pleasure and doesn't consider Star Wars still remains the work of thousands and thousands of people.
People say Lucas is in for the money, obviously, but I can't shake the feeling his passion to meddle in "his" Universe overcame the monetization. Notoriously the way he prophecies Jar Jar in the Phantom Menace featurettes, you can feel the passion of his character coming to life. Jar Jar is still to this day George's favourite character.
TL;DR: Too much control from an otherworldly urge to play god in a collective work as if he built it all by himself.