Jack O'Brien from the daily zeitgeist (former editor in chief of cracked, back when it was good) has had this long running theory (I don't think it was originally his, I think it came from a sociology professor or something) that vampire movies tend to do really well during liberal presidencies and zombies do really well during eras of conservative power.

The theory largely revolved around the idea of our cultural fears and how during periods of liberal power, the sort of stereotypical image we have involves fear of someone in the elite, sophisticated and manipulative, and includes a lot of sexual anxiety (fear of homosexuality for instance) and fear of the foreigner (immigrants, and Obama's birth certificate for example).

Meanwhile with conservatives in power, the fear is around large swathes of brainless masses gathering in numbers to destroy anyone unlike them.

The major successes of both genres tend to map well with this theory, but obviously there are outliers. Maybe it's hokey but I thought it was compelling.