Why is the gothic genre so bloody ?

The gothic genre was essentially the guilty pleasure reading of its day. Victorian society was constrained by the ideals of modesty and decorum, with a good deal of religious piety. Gothic writers rebelled against this culture, notorious for enjoying their 'deviant' sexual natures and drug habits - consider Wilde and Byron. However to earn money they could not simply write explicit pornography, that simply would be censured and be deemed unprintable - so they disguised sexuality in their works, using hints and symbols. Blood took on the role of other bodily fluids, namely semen and menturation blood, suggesting ideas of a sexual nature, and from that implying that the characters - and not just the villains - were sexual. We can consider the role of the Brides of Dracula who became violent, sexualised beings because Dracula drank their blood (i.e. took their virginities but not in the correct, Christian way, on the marriage bed). The few scenes involving them therefore sets up the novel to be centred on themes of sex and morality. 

Blood became a useful prop because sent a clear message that enabled readers to remain removed extreme depravity, yet gain an understanding and pleasure from. Blood was on the darker, immoral side of life but could not itself be censured. Its use is amongst many gothic and neo-gothic works, and has been one of the most successful tropes in literature. Dracula is the only book to have never gone out of print since its first publication (bar the King James Bible).