Hollywood big budget films are at a disadvantage in making horror films. Here's why:
Big movies demand big explanations, which are usually tiresome, and big backstories, which are usually cumbersome. If a studio is going to spend $80 or $100 million in hopes of making $300 or $400 million more, they feel a need to shove WHAT IT ALL MEANS down the audience's throat. Is there a serial killer? Then his mommy didn't love him (insert flashback). A monster from outer space? Its planet exploded, of course (and the poor misunderstood thing probably needs a juicy Earth woman to make sexy with). But nightmares exist outside of logic, and there's little fun to be had in explanations; they're antithetical to the poetry of fear. Link.
There's consensus among Ron Moore and the Battlestar Galactica writing staff that the most terrifying episode of the BSG series was "33" in which the cylons keep reappearing every 33 minutes without any explanation. We don't know how the cylons know where the fleet is, and since they obviously do know, why wouldn't they jump sooner? 33 minutes feels eerily related to the cylons being machines, not humans. So neither the characters, nor the audience, know anything about the cylons—a mammoth question mark hovers over their relentless attacks.
And whenever there's a huge void like that, the brain works double overtime to fill it with every type of childhood fear possible. I'm certain this is why films like The Blair Witch Project, whose miniscule budgets don't pressure the filmmakers to leave the viewer with an adequate explanation, are so wildly popular.
(Thanks to Alex for the link.)