The psychology behind such films is easy to understand. Humans have always struggled between their baser instincts and the potential to improve themselves. Zombies represent the ultimate result of abandoning the latter. Vampires represent unchecked lust and the desire for immortality, but with a price. Both creatures threaten those who would live peacefully, and allow for heroic figures to rise to oppose them.
Zombies also offer the opportunity to wreak wholesale violence without any social repercussions. This has broad appeal in our culture.
Both creatures stem from mythological archetypes so I imagine they will be with us for a long time to come.