The reason for that was a point of argument between the authors of the screenplay Dan O´Bannon and Ronald Shusett (who had previously worked on Ridley Scott´s Alien a decade earlier).
I´ll just quote from this interview with Dan O´Bannon from 2007 (which is a fantastic read btw, especially in the lights of the recent debate on writing fiction, here on this forum)
Ronnie really didn’t know what to do with it, and he wasn’t really comfortable with me writing the whole thing, because…well, call him and ask him yourself. He’ll tell you the deal – the only thing that’s important to him is his name above the title. He brought this over to me and said ‘Do you think this could be a full-length screenplay?’. I know that story well and I said sure I do. I sat down and immediately batted out the first thirty pages and handed it to him.
On the basis of that he said write some more, and I did. I said this is good for a first act, but for a movie you’ve got to come up with two more acts. It seemed obvious to me that the guy has to go to Mars now, because the whole thing is his obsession with being a James Bond on Mars. I said that I thought the rest of the movie should be a cross between Casablanca and a James Bond movie. He said ‘I don’t agree at all, I think it should be a western on Mars’. Y’know, Pancho Villa on Mars. I said that was a terrible idea, and I wasn’t going to write it.
So he went off and found some other schlemozzle to write it as a western on Mars. So after some months he comes back to me saying that he wrote it but that nobody likes it, and he was willing to try my approach. So I wrote another act and got him to Mars, and I continued with that light-hearted…imagine the best of the Bond movies, Goldfinger or something, I continued in that tone.
I figured out what the third act should be, and at that point Ronnie yanked it away from me again and ran off and again got some other guy to write a third act, and everybody hated it. So he came back again and I sat down and wrote the third act at his request. Ronnie took it away and years passed while he ran around town doing deal-making. He finally got the picture financed, and the others now involved completely rewrote the third act into what I consider incoherence. So the first two acts are more or less Phil Dick and me, and the last act is Ronnie Shusett.
As I watch the movie, everything it’s been building up to in the last twenty minutes or more just crumbles into chaos.
credit goes to http://www.denofgeek.com