You see it pretty clearly with Stranger Things. In S1 each character has a specific purpose or role they fill in the story to back up the themes the show wants to explore, and they excel at that role. S1 is great, and they weren't expecting to get an S2. But they did.
Now those same characters, with their specific roles? Well now they need to change (because you're telling a different story), and they aren't a super great fit for the new roles they have to play. It still kinda works, but the show's themes become muddled and you're banking a lot on the audience's love of the characters now. Still a success.
Now we get to S3 and we have to change the characters' roles even more! Entire storylines from S2 have now gone to waste, and many characters are far from their initial roles and don't feel as interesting or compelling as they used to be - because they were never meant to get this far. They're cogs jammed into new and unfamiliar spaces to try and get this machine to keep running. And it lurches and jerks its way forward, but it's a far cry from the efficient, effective show it was in S1.
It's interesting that you bring up Aliens because it's a great example of how a character with previous history can fill a new role effectively, exploring different themes than in previous installments of a story.
But it also highlights the need for major changes - Ripley is the only carryover from Alien, and her character wasn't really fully fleshed out in the first movie, so they had room for her to be whatever they needed her to be in a follow-up. She's not a bad character at all, but she doesn't need a whole character arc to fulfil her role. That's not the case with most TV series because having thin protagonists at the end of a season generally doesn't make for compelling or satisfying TV.
Aliens also works because they introduce new elements to the horror - the thought that there's an intelligence directing the Xenomorphs is terrifying, and the threat it poses to life on earth is almost cosmic horror in its scope. You see that Stranger Things tried to take the same tack, but it was bogged down by its own lore and the limitations and having to work with a whole cast of characters who are experienced in fighting this very threat.