Continuity mistake: Pulse rifles have 10 mm caseless ammo, but if you watch the scene where Vasquez depletes her rifle and then switches to the pistol, there clearly are shell cases being spat out.
Continuity mistake: After the Alien Queen is sucked out of the airlock, Ripley climbs back onto the platform and closes the doors. In a shot from above, her left leg has not yet crossed the mouth of the opening. Cut to a closer shot and half her body is already on the platform.
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.
Continuity mistake: When the team approaches the colony entrance to open the outer door, all the marines are inundated with pouring rain. Yet the moment they enter the compound and start searching, they are all dry.
Continuity mistake: Hicks gives Ripley a wristwatch which is linked to his tracker. If you look at the watch, its face is mostly black with a tiny portion that is clear. However, when Newt has the watch and falls down the tunnel and is wandering around the pool of water before Hicks and Ripley find her, you can see that the watch face has no black - it has a full clear face and appears to be a standard digital watch of the 1980's era.
Continuity mistake: When the crew is having breakfast the first time, at the end of the table is a non-moving kinetic orbit toy in 2 shots, then in the 3rd shot it's moving with no one touching it.
Continuity mistake: When Ripley is driving the APC, the passenger side front tire is on fire and engulfed in flames. When she runs over the alien with the same tire, the flames are completely gone. In the next shot and several shots after, the tire is completely engulfed in flames again.
Continuity mistake: As the ship carrying the marines to the planet where communication has been down for too long, the pilot of the ship says, "Where's the damn beacon? Oh, I see it." But when she says the last bit, there is no vision of the beacon; it only comes into sight a few seconds later.
Continuity mistake: In the beginning, Ripley wakes up in a sweat, practically crying. She rings Burke on the visual phone and she has a droplet of sweat on her nostril, but it's there, then it isn't, then it, is; it comes and goes.
Continuity mistake: When Ripley pulls Newt out of the cocoon, she has cocoon stuff all over her back, but a second later it's gone.
Continuity mistake: In the beginning, when they find the colonists, they have IDs under their skins, but on the read-out there's only about 6 of them, when there should be about 100 of them. Later, on the read-out, we see the right amount of IDs.
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