Like any question about popular culture, this has a couple of answers. First, vampires are ubiquitous as both protagonists and side characters in books, movies, and TV shows because they are proven selling points. This leads to a more specific question. Why do people keep buying and reading stories that repeat these motifs?
The roots of the modern vampire stretch back centuries. In many folk cultures there is a legend of some sort of undead, a spirit that was once human, died, and then became something else, a predator that fed upon the nearby humans, usually the village the spirit had lived in during its lifetime. From the Chinese chiangshi to the Romanian strigoi, these were horrible monsters that appeared in a wide variety of forms and inspired universal terror. Above all, however, these were the creations of peasants.
Bram Stoker’s Dracula changed this perception. Stoker created his monster using some of the characteristics of these rural legends, from the blood drinking and the decline and death of the victims, to the animal forms and distinctive appearance of the monster. But setting them free on a mindless rampage, Stoker blended these traits into those of another stock character of the time, the Rake, as embodied by the scandalous figure of George, Lord Byron. The rake was as dangerous to the members of the post-Renaissance aristocracy as the vampire was to the peasants that served them. He seduced women, leaving them socially and economically ruined. He drank, took drugs, gambled, and fought duels, bringing down fellow men. But his money and social standing protected him to a large extent from any repercussions.
The 18th and 19th centuries produced a raft of popular novels for the newly literate masses. The rake featured in many of them in one guise or another. Often, in the beginning, he was the incorrigible villain, which the virtuous heroine either fell victim to or triumphed over. Later, however, as the Romantic poets became increasingly popular, and the rake became a more sympathetic character. Not that his faults were forgiven or subsumed by other deeds, like the modern anti-hero, but that he was reformed. In a close association with the concurrent ideal of the woman as the base of virtue and piety in society, there came the idea that the right woman could lead such a man out of his evil ways with a combination of allure and determination. An example of this still popular today is the interaction between Maxim de Winter and the nameless protagonist of Daphne du Maurier’s Rebecca. The reformed rake became a hugely attractive figure, as can be seen in the persistence in the popular imagination of the Byronic hero as an ideal of masculinity. What woman wouldn’t want to be able to imagine herself the one person attractive enough, wonderful enough, to bring about such a change in such a powerful figure?
The twentieth century eliminated many of the social structures that provided a framework for the idea of the rake. In western society the idea that one man could ruin someone else, woman or man, is deeply antithetical to our ideas of individualism. So the vampire combines the allure of the rake, of money and intelligence and sex appeal and social grace with the unique offer of immortality. He still offers danger, but more and more is ‘vegetarian’, refusing to drink human blood. Thus is the monster redeemable, whether by a woman or by an ideal, and most of the stories play on this possibility. Plenty of stories still use vampires purely as monsters, as agents of conflict and destruction, but the recent fad has other roots. The face may have changed, but the allure has not.