I think one of the main reasons there aren’t more good prequels is that they work best when recontextualizing the world, characters, or events of the original film in a way that makes them better in hindsight. However, if the film badly needs this recontextualization it usually wouldn’t be a good enough film to financially or creatively warrant any prequel or sequel. This leaves making prequels to successful, self contained stories that have narratively ended which almost always ends up lessening the impact of the original, not improving it.

Also, people are usually really drawn to the characters and world of the original films, which is why they would be interested in making prequels to them, and this means that unless they’re a madman like George they won’t significantly change the world or characters resulting in flat or nonexistent arcs like the monolithically powerful Rebellion in RO compared to later films.