Here's a theory:
There's been a lot of speculation about what is 10.11.12
. I heard that's the first day of Comic-Con, and some are saying we'll be treated to a teaser of Prometheus's sequel at the event.
Others are saying the date might just be the release of the blue-ray, which seems too trite to devote attention to, unless...
...Unless the blue-ray contains the director's cut, which contains 30 more minutes of film that answer every single question left by the original. In theory, a director's cut could satiate people's need for a sequel and could usher in a new home-video sales strategy that could be pretty lucrative: leave enough wonder in the theatrical release to make people want to buy the home release, knowing their questions will be answered in the extra footage.
It may not gross as much as another theatrical release, but they also don't have to incur the cost of actually filming another movie.