He's lost passion for the creature. I still don't get why he wants to keep making these films when he very clearly thinks the concept is played out. If the Alien isn't as effective as it once was, it's because (and we've seen it a million times before with horror) the filmmakers has over explained everything, completely removing the mystery of the thing. Yes, "who and why" are interesting questions, but it's that they remain unanswered that keeps us interested. Giving us all the information makes things familiar and removes the very "alien" essence of the thing.
These films need retconned back to the 2nd film. As far as I'm concerned the only canon films are Alien and Aliens. All do respect to Mr. Scott, but if the Alien is played out, he's at least partially to blame.