From what I understand, the original scope was to run parallel with the Alien universe, not tie directly to it. It's focus would have been on the Engineers. Something which Ridley has always brought up in the past is that none of the sequels explored what/who the Space Jockey was, so his interest has been in that for quite some time. That's why when people want more Alien, it probably bores him.
It's frustrating how things ended up, though, because there is a brilliant movie inside Prometheus.