Then I thought about it a bit more, and this really is the problem with prequels in a nutshell.
Most successful franchises that have an established in-universe history will have what are, in effect, Noodle Incidents that everyone wants to know about.
This is what many prequels try to address, and why many fail.
This is because the original text or film established expectations about the incident that are insanely hard to meet. Add in the predetermined outcome (since the prequel is locked in to the continuity of the original), and you have a recipe for creative disaster.
Good prequels do exist, but I would argue that the quality bar is higher for prequels than it is for sequels.
Questions, comments, examples?