(or "Good, Bad...I'm The Guy With The Fangs")
Let's start this out with a little mental exercise, a little game of "Let's Pretend." Let's pretend you're Marv Wolfman, and you've just been handed the writing assignment to 'Tomb of Dracula'. The series has only been around for six issues, and it's already had three different writing teams; Gerry Conway has set up a team of Fearless Vampire Hunters (descendants of the principals from the novel 'Dracula', primarily), but you've already spotted the problem with centering the book around Fearless Vampire Hunters that hunt Dracula. Namely, it's doomed; they can't win, because as soon as they do, the book ends. (You're probably also figuring out how the book's gone through three writers in six issues.) So what do you do?
You do what Marv Wolfman did; you make the book about Dracula, not about the vampire hunters.
The idea of an ongoing series centered on a villain isn't a crazy one; the public has always had a fascination with the bad guy. Scarface, Dillinger, Jesse James...even Charles Mackay, back in 1841, devoted a chapter of his book 'Extraordinary Popular Delusions and the Madness of Crowds' to the phenomenon of idolizing criminals. Almost as soon as Wolfman takes over the book with issue #7, you can see a distinct foregrounding of Dracula; the Fearless Vampire Hunters never leave the book, but their relationship with Dracula becomes more complex as the series explores Dracula himself.
And what an exploration it is. Traditionally, storytelling engines involving villains try to make them sympathetic, even charming (witness Garth Ennis' 'Hitman' series, where Tommy Monaghan is a loveable rogue who doesn't even kill unless his target is another crook.) But Wolfman goes a harder route, opting for a genuine character analysis of a troubled, deeply flawed individual. Wolfman's Dracula has lost everything over the centuries--his bedrocks of family, faith, joy, and love have all eroded away, leaving him with nothing but the desire to survive...and the knowledge that he was once something more than what he is now. These twin demons drive him to rail against the world like a prison, and yet he fears to escape it; this 'debased nobility' makes for a character so complex and rich that he easily sustains any number of stories told about him (in any number of eras--the companion series 'Dracula Lives!', collected in Volume Four, show flashback stories set in different times and places.)
Wolfman does use one classic trick that every single storytelling engine involving a villain-centered series uses, though. Dracula's enemies are (with the exception of the Fearless Vampire Hunters) worse than he is. Even his arch-enemies agree that it's worth resurrecting Dracula if he'll help out against the sinister Doctor Sun; Blade the Vampire Hunter and Hannibal King, vampire detective set aside their pursuit of Dracula to go after the vampire Deacon Frost; and, of course, when Satan himself is after Drac, it's pretty clear which is the lesser of two evils.
On the whole, it's not surprising that you still see more open-ended series centering on heroes than on villains. It's always easy to root for the good guy, and a lot of publishers feel more comfortable telling stories that teach you to be upright, good, and nice than ones that teach you to seduce women in dark alleys and drink their blood. But as 'Tomb' shows, if you can't be good, you can at least be bad in a cool way.