I think quite a lot comes from teenage nostalgia from whichever era you are from. The original trilogy is not actually all that good, but some people still talk about it as if those movies are the epitome of moviemaking. I think it has a lot to do with the circumstances that people first saw them in. The prequels are also not fantastic movies, but as the people who saw them in cinemas as children grew up they came to be loved more and more. It's not so much the movies themselves that are liked, but the associated memories.

I wonder if in ~10 years we'll have a new batch of people who like the latest trilogy. First ironically as they get through their edgy teenage phase, then unironically as they grow up further and point out some great scenes from between the (admittedly many) terrible ones.