Just take a look at that weekend box office. Sure, the critically panned Couples Retreat came in at #1, earning over $32 million on 3000 screens. But scan down the list and look at #4, which was Paranormal Activity. It earned $7.9 million on 160 screens. That's not a typo. One hundred and sixty screens. If we take the average, Paranormal Activity earned $49,375 per screen, and Couples Retreat took in a paltry $10,666 per screen. That's five times as many butts in the seats for the horror film than for the unfunny comedy (which means that there must have been a lot of empty seats at the latter). There's a simple reason for this: Paranormal Activity is a genuinely scary movie.
The same goes for any of the "body genres," i.e. comedies, steamy films, weepies, etc. If they genuinely work, and genuinely elicit the response that they promise, they will be a hit every time. Horror buffs -- myself included -- probably see more than a dozen new "scary" movies in the theater each year, but it's only once every few years that we actually get scared at one of them. Paranormal Activity achieves this by doing something very simple and not at all new: it doesn't show anything (or, rather, it shows very little). It knows that nothing that can be shown onscreen can equal the fears and nightmares of the people in the audience, and that the fear of the unknown is the greatest fear of all.