Because every major franchise has a linear path.
I could name a few which don't. This answer... doesn't answer my question.
So I'm hoping someone like Alien Predator or PRJ_1990 answers for me.
SiL has summed up some great points.
I think diverging timelines works a bit better in the EU, and technically it's sort of there. I mentioned to you once that the Hish books are basically their own things and unrelated to the Alien vs Predator universe because the backstory is so different. Humans reverse engineer alien tech and they have also made Earth almost inhospitable. And this is 2107 I think?
Meanwhile Aliens as of 2179 shows Earth as perfectly fine, plus some novels were set on Earth past 2107 and humans lived on it just fine. And the new novels mention no reverse engineering, in fact we don't meet our first aliens (Arcturians) until at least the 2160's.
Those are just a few examples of why I think multiple continuities are a bit easier to accept in an expanded universe. And frankly, even those don't really bring back dead characters, the majority of them (especially the new stuff now) try to fit into established lore. They went so far as to change Newt and Hicks after Alien 3 and Resurrection came out so as to keep the stories relevant to the film universe.
The whole point with a singular timeline, Rakai, is to tell a story. Additional stories literally expand the universe as a whole. You may think it is limiting, but I respectfully disagree because just one universe is limitless. And most of the stories are set in the 22nd and 23rd centuries, so they have virtually the entire 3rd Millennium to cover. Not to mention our history too with all the Predator encounters which could go as far back as even prehistory.
it's as versatile as Assassin's Creed, you can take this franchise to any era of time without having to diverge timelines or make things irrelevant. The other point of Alien is it's a dark and gritty universe where literally no one is safe.
She's the main character? Be worried, she could die just as any other character did so far! Might not happen in this movie... might be the next one!
As opposed to what SiL said, "don't worry, she'll be back."
Also many fans hate the idea of making AvP like Star Trek. This is something I've seen people (even you occasionally) use, where they would say "it's too much like Star Trek", "this peace conference between Predators and humans is too Trekky", "this translator, it makes AvP more like Trek..."
Well, Alien and Predator lack something Star Trek has in spades = official and in-universe diverging timelines.
I think this should be left out of the movies. Let the films have a proper timeline, a proper history for us to enjoy and catalogue. The EU isn't as "in your face" about diverging from something established in a prior story. And those that diverged too much weren't as well received. Who actually remembers... no, who KNOWS of the Hish besides hardcore fans? Yet even the most casual people know "Yautja". Heck, I got the Alien vs Pinball game, and noticed "YAU - TJA" written on the pinball table in the AvP map.
That's because the stories with the Yautja are much more beloved and known about due to the fact they had a respect for established lore and fit in quite well.
It's not like the new ST films where out of nowhere, an Engineer ship just comes back from the 41st millennium and f**ks up the whole current flow of time.
Also, look at the reaction people have to "The Predator" and compare it to the reactions of "Alien 5". It seems obvious that the majority want a singular timeline. Black says he will continue this as a sequel because there's so much to explore in what's established. Whereas Bomkamp just wants to give his version of a "true" ending to his favourite character.
It's acceptable for Godzilla, Star Trek, TMNT and Terminator because those have in-universe time travel, in-universe paradoxes, in-universe official diverging timelines. I'm not a fan of the concept, but I can accept it in those franchises (and I like those listed as well, I grew up with Terminator and Godzilla).
But Alien and Predator are grounded, pseudo-realistic settings like Assassin's Creed. We have yet to find a diverging timeline in real life lol. Not every story needs one because frankly, just "one" is enough. Just one can make this franchise bigger than two or three.
So, that's my essay on why I think one timeline is enough.