It may be because Alien is a survival horror movie in space whilst Aliens is an action movie.

Also Aliens defined almost every action movie trope and convention there is.

Also Aliens is an absolutely superb, seminal movie.

Also it's probably fair that a single alien on a ship of what, seven people, who have no idea what's going on, is able to pose more danger than a bunch of aliens can to trained, equipped space marines who have some knowledge of what they're getting into.

Also hardly anyone survives Aliens so I can't see how it can be said they're not dangerous.