The lack of humans. I know there’s a few stories where humanity has died out and some other kind of being has taken their place, but I don’t mean that lack of human - humans still impact such a story. Rather, I’ve always found BIONICLE fascinating as it’s the only (serious) story I know not set in some variation of our world.
There are no humans in any way, and likewise there also are no ants, horses, cats, etc. Sure, there are creatures inspired by real-life animals, but they’re not those real-life animals. Sure, Glatorian/Agori are reminiscant of humans, but they’re also not humans or related to humans. The world is completely fictional.
You don’t find that with superhero comics and movies - they’re set on Earth or off-planet but still in a world where Earth exists. You don’t find that with Pokémon - humans still exist. You don’t find that with HF - Earth and humanity exists there. You don’t find that in Ninjago - sure, they’re minifigs but they’re still meant as “humans.” You won’t find that in Bugs Life - that’s still about “real” bugs, on planet Earth, in a universe where Earth exists. You get the idea, I hope.
Bionicle was an escape from humanity, in a way nothing else was - and I think that’s the primary reason that kept me interested in it, and it is something that can be taken in so many different directions. I wish more would do so.