When Lucas was working on the original trilogy, it wasn't just him that worked on it, but a whole team that worked alongside him. For that, his crew was not afraid to give Lucas some constructive criticism, and tell him what they think would work and what they think wouldn't. That type of peer review is how the original trilogy ended up being as good as it is.
Fast-forward 16 years after Return of the Jedi came out, and Lucas was mostly credited as the mastermind behind Star Wars... and when the prequel trilogy was in development, he worked with a much younger crew. Because they weren't with him in the original trilogy, his crew saw Lucas as this legendary filmmaker, and for that, they wouldn't give him any constructive criticism, because when you're on top of the world like he is, you just simply can't question his genius. That's where we ended up with Jar-Jar, and the bad writing, because everything was being done by George Lucas. It wasn't peer reviewed like the original trilogy is, and that's why I think the prequel trilogy ended up being as awful as it is.
