What is it about the fantasy-horror genre that gives it such widespread and enduring appeal?

During the mid-15th century, Vlad the Impaler ruled the southern region of modern day Romania, (a territory then called Wallachia). Tales of his atrocities were circulated by the German Boyars of Transylvania and became popular reading all over Europe. Even decades after his death, private press owners were guaranteed to make money reprinting these Dracula stories. When Bram Stoker resurrected the character in his 1897 novel, Dracula, he had an instant best seller. In over a hundred years, the book has never been out of print. The following century saw the birth of Tolkien's Lord of the Rings, which in numerous polls (in both North America and the UK) was voted the book of the century, and its author the storyteller of the century. The success of J.K. Rowling's Harry Potter and Stephenie Meyer's Twilight series only confirm the popularity of the fantasy-horror genre. What is it about fantasy and horror stories that give them such widespread and enduring appeal?