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: 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.
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.
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