Maybe it wasn't your cup of tea and didn't make you jump. If that's the case, fine. However,
Alien built the house that a lot of the genre lives in. It's pretty much the standard bearer of sci-fi horror for the "Seinfeld isn't funny" phenomenon (
link).
Sure, it's not as gornish as a lot of modern horror. IMHO, a lot of modern "horror" is a terrible excuse for torture porn. There's some good stuff (
The Descent was brilliant), but the tendency is to equate bizarre, spectacular and horrible killings with "good" horror.
Alien, though, relies on suspense rather than effects and splatter shots because it's over 30 years old. The killer is barely seen until the end of the film. Its capabilities are unknown. It can't be negotiated with or understood, and it sure as hell can't be faced down without weapons. Worse, it's
smart.The most exposition about it that the audience gets is from Ash. What's scarier - the monster you know, the one you don't, or the one you thought you did?
Especially after you learn that Ash, and therefore the Company, knew all about the damn thing, probably before the crew even set off on their mission.
I'm not sure what qualifies as scary for you. However, I would bet that if you'd walked into that movie theatre in 1979 not knowing what was coming, you'd have gotten a few good scares out of
Alien.