BWP came out in 1999 so I assume it was a couple of years later when it was on Channel 4/Film 4 (so I'd have been 11/12?). It seems silly now for me too but all the grainy footage spooked me as a kid. It felt unsettling and claustophobic.
I think that's the thing with horror - there always tends to be something that makes people feel scared. For some kids it'll be ghosts (especially in and around the home), for some it'll be demonic visions, etc etc. Whatever relates to existing fears.
It's why I think stock stuff like Zombie or Werewolf movies, while scary, doesn't unsettle people the same way. You don't tend to encounter or think about them much in daily life. They only exist in the movies (or at least I hope).
No idea if I'm being helpful here for the point of your thread but the idea behind horror movies interests me a lot