Zombies are generally an easier metaphor, as they represent the unwashed, consumistic masses. Thus the common trope of the mall/urban setting. And that clicks nicely with many players' pent-up aggression against the society at large.
Vampires are a much more mature beast, as they classically stand for the seductiveness of the animal and sexual part of the human psyche. Even in their modern incarnations ( from Anne Rice to the Twilight series ) they are mostly presented in the light of the conflict between (sexual) urges and the ethical or moral constraints.
Vampire: The Masquerade (the 1991 pen and paper RPG and the derived works) was actually quite interesting as it moved the source of conflict towards different kinds of "essences" of which blood was the obvious metaphor. Not only life and sex, but also artistic rapture, political power, spiritual illumination, magical knowledge. Its tying-in with jewish and christian mythology once again denounces how basic the underlying themes are.
In the end, though, this finer texture is much harder to turn into palatable games as it would lead to tackling the issues of subtler, inner conflicts. And games haven't been -generally speaking- good at subtlety, just look at how potentiually interesting themes in Bioshock devolved into its gameplay.







