He was probley too much of a big/upcoming name when Alien was planned. I mean think about it? you get offerd the 2nd Star Wars movie because the first one was such a mayjor hit, and you probaly be getting a shit load of other scripts shoved in your face, and then someone else says we have this low budget B-movie type of horror sci-fi flick called Alien with some unknown director and a b-movie plot... What would you go for? I know the likes of Tom Skerritt, Harry Dean Stanton, John Hurt, Ian Holm were well known at the time, but the had nothing to prove because the already had street cred. I would put the likes of Harrison Ford back then on the same lines as Christian Bale is today, a new super star were everyone wants to use him in there latest mega block busters flick.
But i do belive if he got offerd Alien afew years after the Star wars buzz died down he may of went for it, to try he hand at something differnt.