One of Ridley Scott's most exclaimed science fiction movies is Alien. It's also one of the few movies that does not have a city scape. When involved in the re-release in 2003 he started to want to direct another Alien movie. It's states on his wiki though that he wasn't happy with the Director's Cut because he found the original pretty flawless. So why would he make a prequel then?
It's not actually a prequel, but rather is set in the same universe but earlier in the time line. He made the coice to not make a direct prequel to essentially avoid being repetative in the material.
The film began development in the early 2000s as a fifth entry in the Alien franchise, with both Scott and director James Cameron developing ideas for a film that would serve as a prequel to Scott's 1979 science fiction horror film Alien. By 2003, the project was sidelined by the development of Alien vs. Predator, and remained dormant until 2009 when Scott again showed interest. A script by Spaihts acted as a prequel to the events of the Alien films, but Scott opted for a different direction to avoid repeating cues from those films. In late 2010, he brought Lindelof onto the project to rewrite Spaihts' script, and together they developed a separate story that precedes the story of Alien but is not directly connected to that franchise. According to Scott, though the film shares "strands of Alien's DNA, so to speak", and takes place in the same universe, Prometheus will explore its own mythology and ideas.