It’s not clear what has changed in the last couple of months to change the director’s plans so drastically.
Scott does talk a bit in the Yahoo interview about how Prometheus was not as warmly received as he would have liked, but certainly, he’s known that for some time.
And while even yesterday we were expecting this next Alien film to be just another in an ongoing franchise, now it appears that this script being written could potentially be the last one that Ridley Scott plans to work on.