William Goldman addresses this phenomenon in his screenwriting book, “Which Lie Did I Tell?” — well, not prequels exactly but backstory. One example he cites is a scene between Casablanca’s Rick Blaine and Captain Renault.

Captain Renault: “What in heaven’s name brought you to Casablanca?”

Rick: “My health. I came to Casablanca for the waters.”

Captain Renault: “The waters? What waters? We’re in the desert.”

Rick: “I was misinformed.”

Goldman points out how much more effective this evasiveness is than if Rick had detailed the series of events that led him to Casablanca. If he’d said that his marriage failed or that he lost his job, it wouldn’t have been as compelling as this avoidance, which forces the viewer to wonder. That mystery is potent.

Prequels fail, in my opinion, when their chief purpose is exposition.