I think it's likely that this happens in three ways:
Alien 5: Probably because Sigourney Weaver and Neill Blomkamp want to end the story of Ripley (and have raised a lot of Hype in fans). In addition, Fox seems to have the idea that Ripley is the soul of the franchise.
Standalone film (ala Predators): Probably because they have no idea how to continue the story of Ripley. That is, short and simple; a movie with new characters, new establishment and new story.
Reboot: Something that Hollywood loves, and a good strategy to attract the public of the new generations in order to make more money.
BTW, retcon Alien 3 out of existence is possible, but for now it is just speculation (like my words anyway XD)