Factual error: Master Sergeant Alexander "Al" Apone U.S.M.C. ranking was wrong on his uniform. His uniform showed a U.S. Army Sergeant First Class rank. (You can see this when they were loading up on their APC.) The correct ranking should have been 3 chevrons up, crossed rifles in the middle, and 3 rockers on the bottom. *Yes, in the future, the ranking system could have changed.*
Revealing mistake: When Ripley enters the elevator to go down into the hive, her pulse rifle/flamethrower combo is a stunt prop. The giveaway is the pulse rifle's front is blank (literally just a flat surface with no detail and no holes for the barrel or the grenade launcher). It's particularly obvious in the newer HD versions.
Answer: It really was all down to James Cameron having already written the script and proving himself capable of directing with 'The Terminator.' It was just a quicker, easier, and almost certainly cheaper decision to let him direct his own script rather than get someone else, even Ridley Scott. While the producers had wanted to make an 'Alien' sequel almost immediately, at the time the head of 20th Century Fox didn't want to pursue it fearing it would be seen as an obvious cash-in and flop. When a new executive at the studio came in a couple years later, the project was put back on track, and I believe Cameron was the first to be approached to write the script.
TonyPH