Continuity mistake: In the scene close to the end of the film when Ripley is getting the weapons ready in the 2nd drop ship to rescue Newt, she slams in a new ammo clip into the pulse rifle and the counter reads 95 rounds. Moments later when she is in the elevator descending into the aliens lair the counter is reading 42 rounds, even though she hasn't fired a shot.
Continuity mistake: During the Cargo Loader vs. Queen fight, Ripley grabs the Queen and it closes around her neck. In the next shot it is closed around its head. It changes extremely quickly; therefore it couldn't have changed position.
Continuity mistake: At the beginning of Aliens, when the door is cut through, the markings and lettering on the transparent cover of Ripley's suspended animation pod are different from those that were on it when she was asleep at the end of Alien.
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: When Ripley and the marines arrive on the planet, it is pouring with rain. But a minute after being inside the buildings, they are all dry.
Continuity mistake: As the marines first enter the med labs, Drake at the front, there are some neon tubes hanging from the ceiling that slowly rotate to face the camera. As the angle changes and Drake goes round a corner, the neon tubes rotate to face where the camera previously was again in the background.
Continuity mistake: There are 2 squads of marines in the film, 9 in total. (Not including Gorman or Bishop, and also not including Ferro and Spunkmeyer who stay in the dropship and do not enter the compound.) Squad one is Apone, Vasquez, Hudson, Crowe, Wierzbowski. Squad 2 is Hicks, Frost, Dietrich, Drake. But as the marines enter the compound you clearly see 5 marines running up to the door, and shortly after, Apone shouts for second squad to move up. As they move look closely and you will see 5 more moving up. 5+5=10, who is the mystery marine?
Continuity mistake: In the extended edition of the film, Ripley, Gorman, Burke, and Bishop meet up with Hicks at the entranceway. It is raining heavily and Ripley's hair gets soaked. But when she is inside a few minutes later, it looks as if she has had time to dry her hair thoroughly between now and then.
Continuity mistake: As Ripley tucks Newt into bed for a nap, the angle of the reading lamp over the bed changes about 45 degrees following their close up discussion of the scary dreams of the doll.
Continuity mistake: When Ripley sees the Queen on the Sulaco, she tells Newt to run. In the next shot of Newt getting up, she has moved further away from the step.
Continuity mistake: At the very end, when Ripley is closing the lid of Hicks's sleeping capsule, we see a shot of him and the bandage almost covers his left eye. In the close-up shot of the lid closing, the bandage is further up on his forehead.
Continuity mistake: When you see the attack aircraft flying towards the planet from the front you can see the pilot Ferro in her dark glasses sitting on the right side of the cockpit, but in all the other close up shots she sits on the left side with Spunkmeyer behind her on the right.
Continuity mistake: Hicks has a heart painted on his armor with a latch locked by a padlock. When Hicks and Ripley enter the elevator after losing Newt, the padlock is missing. After the Alien attack, the padlock reappears.
Continuity mistake: When Bishop is doing his knife trick on Hudson, his hair changes significantly from one shot to another, especially on the top and around the ears. In addition, at the end of the scene, Hudson is given a meal that was taken off a heater nearby on the table. That meal was apparently not placed back for some of the closeup shots, as it is missing from the heater. Finally, the time shown on Bishop's wristwatch seems to change radically about half-way through the scene.
Continuity mistake: When Ripley and the marines go to where the colonists are, there is a ladder in front of the vehicle that was driven inside; the ladder would be in the way if the vehicle was driven further in. But when all hell breaks loose and Ripley decides to drive the vehicle further in, to rescue the marines, the ladder is gone - in fact, a lot of things look different the second time round.
Continuity mistake: Just before the marines are extracted, Hicks tries to close the APC door. An alien stops the door and tries to get inside. Hicks picks up his shotgun and jams the muzzle into the alien's mouth and fires, killing the alien. The alien falls away, they immediately close the door and the APC begins to back up. The camera shot changes to an outside view of the APC. If you look at the area as the APC backs up, you'll see falling debris but no dead alien. It's doubtful the marines would've pulled it into the APC as we don't see it again, or that they would've kicked the corpse out really far due to the acid.
Continuity mistake: In the original "Alien", Ripley dons a space suit in the shuttle, in preparation to decompressing the quarters to the hard vacuum of space. The helmet has no light on its top. Subsequently, in "Aliens", the opening shots pan through the shuttle chamber, and Ripley's helmet sits there...with a light on its top.
Continuity mistake: In the scene where the drop ship is about to leave the main ship look at the picture on the side of the ship, just to the left of the pilot's window. There is a symbol of an eagle. Note which way it is facing. Later when the ship is on the planets surface and the marines are leaving it, the eagle is facing the other way.
Continuity mistake: When Newt is caged in the resin, the cloud of steam around the egg completely disappears when Newt is seen from the p.o.v. of the egg.
Continuity mistake: At the start of the film when you see Ripley in her stasis capsule the glass is clear, but when the salvage men enter the shuttle craft the glass on the capsule is covered in frost which one of the men has to wipe away in order to see her face.
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