Lucas was excellent at big picture things, but should have been kept out of the details. Particularly dialogue
Way I heard it, he was well aware of that and actually tried to get people who would slap him down as necessary, but didn’t manage it.
It was also the result of inexperienced actors.
In the OT the actors would regularly be like “I don’t think this line sounds good let’s with this instead”
But in the PT everyone just went along with everything due to lack of experience and not wanting to be hard to work with/think they’re better than the director
That’s just about the opposite of the relative experience of the actors involved in both trilogies.
The original trilogy was filled with almost completely inexperienced actors. The prequels were mostly cast with experienced performers, excepting the child actors of course.
The real difference is that Lucas didn’t actually direct most of the original trilogy in the first place.