And when you know that the character HAS to survive because you've already seen their death scene elsewhere, sometimes there's a feeling of "this particular story doesn't matter".
Like Anakin Skywalker's hour long pod race, for example.
There is a technical name for this but I can't recall it and I couldn't turn it up in a Google search. It refers to a "flashback" type of story where you know how the ending has to turn out one way or the other (death or survival), and the author tries to build suspense anyway, but the suspense fails, because you know the character can't be dead (or will die).
The Anakin pod-race is a good example and it's why certain aspects
of prequels or flashbacks don't work. This is not to say that prequels/flashbacks cannot work (or stories about a now-dead character from years ago), but, the writer has to be very clear in his own mind about where suspense comes from. The Anakin race doesn't work well because it revolves around the question of Anakin's survival. We already know he's going to be Darth Vader, so the question of his survival is irrelevant -- and the pod race ends up being non-suspenseful.
On the other hand, there are elements of genuine suspense/interest in the prequels that revolve around "how did the world get to be where it was when the first SW movie came out?" HOW did Vader fall to the Dark Side? That is interesting. We know he will fall, but not how, and lots of people wanted to see that moment.
So the writer has to realize what he is doing when he does a flash-back scene or a story of a character whose fate is already known. Let's say they kill off Wolverine but then do flash-back stories of him (to keep selling comics). The writer has to realize that he cannot use any sort of story in which the suspense revolves around "will Wolverine survive?" because we already know Wolverine's fate. Instead, the story suspense must be based on characters whose fate remains UNKNOWN. This is why in flah-back stories, one introduces new characters who weren't seen later on, so we don't know what will happen to them. A good example of this is Short Round and Willie in Indiana Jones and the Temple of Doom. We know Indy survives (because Raiders, which came out first, takes place a couple of years after ToD), but since Willie and Short Round don't appear in Raiders and aren't referred to one way or another, we don't know if they will make it. Hence, the suspense revolves around THEIR survival, not Indy's.
So the point is that yes, one can do prequel or flash-back stories and can do them well, but the writer has to change the way the stories are usually written. A flash-back can't attempt to evoke suspense around the question of whether a character who appears alive LATER will survive or not. The suspense has to come from somewhere else, or you get a really flat, boring story.
And that's why the pod-race scene doesn't work so well in Ep. 1... it's all about "will Anakin survive?" but we already know he does.
Now, if we had not heard the name Anakin Skywalker before... if we did not know that THIS KID was going to become Vader, if Lucas had somehow hidden that from us (let's say, a whole family of Skywalker boys, and you don't know which one ends up being Vader), then there would have been suspense, and that scene would have worked (better, at least). But because there is only one Skywalker kid, and we already know Anakin = Vader, it was the suspense that was killed instead of the character.