Horror and mystery
I generally dislike "prequels" because they almost always seem to undercut their franchises. Mystery is delicious, including the mysteries that lurk within coherent but unspecified backstories. Anakin Skywalker's fall from grace was a lot more interesting before we actually got to watch it, and I suspect that would remain true even if the "Star Wars" prequels had been better films.
Horror is particularly dependent upon mystery. "Prometheus" sins against that, by providing a labored explanation for the beautiful, chilling, enigmatic sight of that disturbing crashed ship from "Alien" and its long-dead pilot (who was not a big bald human-looking guy wearing an elephant-shaped mask!) That ship was someplace Man Was Not Meant to Go, but we went there, in defiance of a warning that should not have been ignored. All else should have been left to our imagination. Really, the alien eggs (not containers full of black goo!) packed neatly in the hold of that ship are reasonably consistent with the idea of a bio-weapon that got out of control en route to deployment, but the point did not need to be made at such length.
(As an aside, the ship in "Alien" was discovered on a different planet than the one the "Prometheus" visited - a subtle point "Prometheus" really could not have made more clearly, because it's a prequel, but the code designations for the two planets are different, and they don't look the same. This raises the possibility that the ship in "Alien" was en route to a different destination - maybe the Engineers decided to eradicate every race they had created, across the galaxy. That might be an interesting notion, if "Prometheus" didn't suck worse than an off-the-record press briefing from Eric Holder.)
Look at what Rob Zombie did to poor old Michael Myers in his "Halloween" remakes. Myers went from the ultimate force of inexplicable evil - so named by his own psychologist! - to a tortured man-child killer with an elaborate backstory. Horror film makers have spent decades misunderstanding the reason John Carpenter's ending for the first "Halloween" worked so well - the enduring mystery of this dreadful Shape (as he was referred to in the credits) cemented by his disappearance after taking six rounds in the chest and falling out of a window. "What was he?" is supposed to be a lingering question in the mind of the audience, not the beginning of a studio pitch meeting for an unnecessary reboot.
There was something of Lovecraft's idea of cosmic horror in the original "Alien" - fragile humanity, comfortable enough in its technological superiority to begin using deep space like an interstate highway, taking the wrong damn off-ramp and discovering just how huge and dangerous the universe really is. James Cameron's sequel is a quality film and an action/suspense masterpiece, but I was never entirely comfortable with the way it de-mystified the Alien a bit too much, turning them into a hive of savage animals with an unsettling degree of low cunning, rather than the enigmatic force of cosmic savagery encountered in the original film. Having watched "Alien" many times in my youth, and read the novelization (different in some respects from the shooting script) over and over again, I always wondered just how smart that terrible creature really was. (The novelization says it was intelligent enough for Ash, the robot, to begin communicating with it.) The unknown remains a powerful component of both fear and intrigue, which in their purest forms are closely related.