Once upon a time, science fiction and horror were both pulpy genres, in both books and movies. But now, science fiction has its own bookstore section, and SF films are huge blockbusters. How did SF beat horror, Cinematical asks.
I don't really think science fiction has defeated horror. The trouble we have with these genres nowadays is that they make sharp distinctions were there are none. Alien, The Thing, Terminator and Matrix all had horror elements in them.
Horror often starts off with creatures that are tougher than humans, that behave in ways that are hard to understand and that kill people right and left, usually in violent ways that invoke body horror and revulsion. That they are robots and aliens or ghosts and vampires is largely irrelevant. I could argue that the basis of horror is ultimately a pessimistic one where everyone is doomed and the universe is not merely indifferent but actively hates and conspires against us. By that definition, even some Shakespeare tragedies can be considered "horror."
There is always a point in any horror story where the protagonists realize that everything they assumed up till then is completely wrong and that they are royally screwed. It doesn't have to involve supernatural elements—madness and psycho killers can do it too. Or aliens and robots.