There's something about that word which just grips the hearts of many. When you're in a moment of outright terror, that's the word to describe it. Horror is just an adjective to describe terror, so it's not really that hardcore of a word. But terror is a real thing. It's something that causes fear.
I feel that the word horrifying is far more scary to most people than the word terrifying. Perhaps the reason is because our culture has used the term "horror" more readily, especially in terms of genre for such things as movies and books. Because we have such a close association to this word, established by popular culture, it immediately draws images to mind in a way that "terrifying" does not.