Ripley and Hicks in Aliens (1986) | Image: Twentieth Century Fox
The number of factors that are involved in whether a movie character is written out of a subsequent script can be huge. Maybe the actor was a nightmare to work with and no one (especially the director) wants a repeat performance. Maybe the actor asked for a huge salary hike and the franchise found the desire unrealistic and were unwilling to meet the demand. Or perhaps the character simply doesn't make sense in the context of the new script, and the principals want "new blood." It could even be a combination of all of the previously listed factors and some in addition.
Ultimately, whatever the reason for the character/actor taking a powder, the audience is presented with a reason for the absence. Sometimes the explanation runs the range of "not well fleshed out" to "patently ridiculous."
7) Newt, Hicks and Bishop in Alien 3:
Ripley’s entire ragtag family died on impact when the Sulaco crash-landed onto a prison planet—or more accurately, they were impaled (Hicks), drowned (Newt) and deactivated (Bishop). They are eulogized thusly:
“Why? Why are the innocent punished? Why the sacrifice? Why the pain? There aren’t any promises. Nothing’s certain. Only that some get called, some get saved."
This article at i09 lists 29 instances in which characters were written out of movie sequels. Well played, plausible, improbable or pathetic? You decide.