Thing is, in the OT they had just started to tell the story, so no backstory is needed. When you’re listening to someone telling a fairy tale, and they start saying: “Once upon a time there was an empire…” you don’t immediately ask where did the Empire come from, because it’s a given now. You know it’s there, it’s part of the story.

Now, if in that story, at the end of it the Empire is destroyed, and then a couple of years later someone decides to tell you a sequel to that story and start by saying: “once upon a time there was an empire…” again, it’ll leave you wondering: “wait a sec, wasn’t the Empire destroyed?? How did this happen???” And if the person telling the story doesn’t give you an explanation I think it’d be reasonable to think it’s some sort of plot hole in the story.

The prequel trilogy wasn’t needed because it’s a prequel. But if the OT had been released after having 3 movies set before the PT where a bad Empire was destroyed, and you basically have another bad Empire without explanation, people would be upset, and the PT would be necessary.