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.