If you look at the PT vs OT, many things were different when Lucas made the PT compared to the OT. He had Kershner and Marquand direct the OT sequels, while he directed all 3 PT movies. He had Kasdan write the OT sequels, while he wrote all 3 PT movies. He had a hands-on Producers in Kurtz who didnt cater to everything on Lucas, whereas Rick McCallum was a total lacky to everything Lucas wanted.
The one thing that didn't change with the OT and PT is John Williams, hence why the music for all 6 movies is great.