Well, it's probably the fact Marines populated almost every piece of Aliens EU. Most of the games involve you being a Marine, running around, shooting them damn Aliens. Even comics for a loooong time were published under the "Aliens" label which gotta be saying something
I see your point though. I'm myself not the biggest fan of people trying to push Alien to the status of an arthouse masterpiece while at the same time dismmssing Aliens as "just another generic sci fi action 80s movie" and "CaMeRoN tUrNeD eM iNtO bUgS aRrRgH !"