Continuity mistake: When the loader is dragged into the airlock, the yellow caution light shatters. The light is fully intact in the next shot.
Continuity mistake: From Director's Cut. The scene where Al is being quizzed about the ownership of the discovery of the object discovered by the "Mom and Pop" survey team tells us that a call has been put in by that team to the base. But then when they discover the ship, where is the call? They explicitly do not call it in. Now this is not just a general enquiry because they were sent out there two weeks ago to a specific grid reference. "He says he's onto something." (00:16:34 - 00:18:38)
Continuity mistake: When Ripley loads the magazine into the gun as she prepares to rescue Newt it says 95 rounds but when she is going down the elevator shaft it says 42 without her firing a shot.
Revealing mistake: The "mission time" clocks on the video screens are very inconsistent most of the time they're seen. At several points they jump back and forth from 00:01:31 to 00:01:32 then back again even though all of them are supposed to be synchronized.
Other mistake: When Hudson is looking for bios on the screen, it shows them on level 3. But when Frost falls down the hole after being hit with a flamethrower, he falls down a number of levels.
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