Quoted:
Its a fannon thing. Its not the cannon name.... When saying it, the characters are literally just saying "Alien" and not a specific name.
"Fanon", "canon". And it's irrelevant -- all the other names are
even more generic and difficult to recognize. "Xenomorphs" gets your nearly universally understood meaning across in three syllables. Saying "the alien" or "an alien" or "organism" would be much more ambiguous, and saying "the alien from Alien" or "H.R. Giger's alien" or any other name takes longer and makes you sound like a dingus.
There is no confusion about the term "Xenomorph". To my knowledge, it hasn't been used in any other ridiculously famous movie to name the antagonist, and I've never witnessed anyone write it or say it to mean anything but the bugs from Aliens.
"Xenomorph" is the most concise, most obvious, most interesting, least phrasey, and therefore best name for the-bugs-from-Aliens.