By giving into in fans' desire to flesh out a fictional world, films like these sacrifice the sense of wonder that made the original work so enjoyable.
In Prometheus, Ridley Scott's return to the Alien franchise he began in 1979, a crew of scientists and corporate drones head off to the far reaches of the universe in search of an extraterrestrial race they believe is responsible for the creation of humanity. It's a movie about hubris and the realization that seeking out answers to existential questions can lead to horrible discoveries. Funnily enough, this theme doubles as a sort of meta argument against prequels like Prometheus, which eagerly offers up a backstory for elements of the previous Alien films that never needed to be explained.
Prequels take one of the most engaging and imaginative aspects of fandom—obsessing over the inconsequential details that give a fictional world its character and texture—and move it off of message boards and onto Hollywood back lots, turning it into something poisonous to the art of storytelling. Plot points become pedantic info dumps, drama is diminished by the audience's awareness of stories taking place in the future, and writers and filmmakers end up rehashing the flashiest superficial elements of their source material while draining of it of mystery and metaphor. Whether it's Prometheus, the Star Wars prequel trilogy, or DC Comics' thoroughly unnecessary line of miniseries filling in the back story of Alan Moore and Dave Gibbons's pointedly self-contained Watchmen graphic novel, the projects have the visual hallmarks but cast aside the tensions, themes, and tone that made the classic works resonate.