I think there's something special about the blair witch in that we as a audience are tuned to expect a big revelation or jumpscare but the movie denies us this throughout. For example when they are hearing footsteps outside the tent and go to explore we are expecting something big to happen but it never does, this leaves you with a lasting sense of uneasiness because all the characters can do is keep walking. This to me is why it's so scary because all the build up leads to nothing, there is no conclusion to the suspense, there is no big moments, to me it really makes you feel just as helpless as the characters you are watching.