Ridley publicly stated numerous times that he felt hesitant about doing another SF film; he felt all possible SF stories have been done, and there was 'nothing new' in the genre.
Along comes Prometheus, from what we can now see a visually stunning, but albeit somewhat flawed film. Not what i personally expected from Ridley.
So I have a theory:
- Maybe Ridley does not see Alien as his premier work – his master piece is Blade Runner.
- To Ridley, starting a new Prometheus franchise and filling in all Alien plot holes is less important that making his next masterpiece, Blade Runner 2, a stunning final masterpiece
- After 30 year SF hiatus, Prometheus is a trial run to see how to engage the next generation of young audiences with retro material – that's why the long line up of characters, trailer overload and web based movie tie-ins. Movie making in 21st century style.
Or maybe the explanation is way simpler - Prometheus is a layer cake – it has to be watched a number of times to fully appreciate the multiple plot arcs.