The Gang's All Here
Of recent vampire pop-culture phenomena, very few have featured just vampires. Let the Right One In and its American remake, yes, but Twilight is chock full of werewolves. Being Human hits three supernatural species right off the bat. And by the end of last season, True Blood was a cryptozoological United Nations. Vampires now have to operate in context, as opposed to being placed in simple opposition to humanity, and as such they don't get to do scary vampire things as much. It becomes a comic book scenario - vampire vs. werewolf is a lot closer to Superman vs. Batman than you'd think - and that reduces all parties to glorified action figures. There's no scare there, because it's powers against powers, not frail human flesh against something terrifying.
Zombies, on the other hand, travel in homogenous hordes. Go up against zombies and you're up against zombies - a clean, clear distinction where the lines are drawn between the living and the dead, and the words "Team Edward" will never be uttered.
Mythology Is Hard
Vampires are killed by A) garlic B) crosses C) any religious symbol D) a stake to the heart E) Joss Whedon dialog F) bullets with liquid sunlight in them or G) cheese. Apart from G (though to be fair, a good garlicky chevre might do the job), any and all of them would work, depending on whose vampires you were playing with. As vampires blossomed in popularity in the post-Anne Rice era, the number of mythologies around them exploded. Everyone reinvented vampires to suit their needs; the cultural shorthand as to what vampires were and how you dealt with them was lost in the muddle.
Zombies, on the other hand, come in basically four flavors: fast and made by science (28 Days Later, Left 4 Dead), fast and made by magic (Day of the Dead), slow and made by magic (Romero's Dawn of the Dead) or slow and made by science (The Walking Dead). That's it. They're fast or they're slow, and that's pretty much all you need to know.
You get the same thing socially. Vampires have been subjected to ever-increasingly complicated histories and genealogies of late. Clans, lineages, origin myths, rituals, rites and a creepily made-up Dakota Fanning - all of these are red meat for the dedicated vampire fan who wants to dive into the subject matter as surely as a wannabe Jedi grows a rattail. And there's certainly monstrous hay to be made on the notion of centuries-old rituals and unfeeling, predatory beasts hidden behind a veneer of ancient civilization.