Why didn't it happen? The producers simply didn't like the script, a result that happened with several other writers -- including Pitch Black's David Twohy and The Hitcher's Eric Red.
Will it ever happen? Doubtful. The Alien franchise has already moved on to a fourth film and two Predator spin-offs, all without Gibson's input. I suppose it's possible he could be approached again to work on an Alien film, but with plans for an Alien prequel scrapped in 2011, that would be years away, and the storyline would probably have to be quite different from what he previously wrote.