Why was Stoker's Dracula so revolutionary for its time and why did he write it?
Bram Stoker, like many other Victorian-era writers, reflected the behaviors and beliefs of London society as a way to let society see itself through an alter mirror, and to cast his own arguments in favor and against the memes of the time.
In the case of Dracula, this happens even in the fact that he setting of the story is mainly in modern London, whereas the historical facts of the story remain at Eastern Europe. This gives Stoker a chance to use his reality to create an alternate one which reflected the opposite:
a) The religious Victorian fervor and their anxiety of Eternal Life in heaven is an antithesis of the vampire's never-ending life and return from the dead; Yet, the meme of Christian salvation is also resounding as it was in London society at the time.
b) the Victorian supression and denial of sexuality in women is also an antithesis of Dracula's openness in female sexuality. The females who are transformed in to vampires become vixens; Stoker's description of them throughout the story denotes that he was quite excited about them himself.
c)Sex is open, neccesary, and exciting in the novel. This contrasts with the prudish, proper, virtuous, and "holier than thou" attitude of Victorians.
d) The dark elements of blood, death, pain, despair all combine to create an ambience of Gothic tragedy. This was also a way to create a world alternative to that of modern society, and a way to see it for what it was.
Basically, Bram Stoker, like many authors of his generation, took society, and exposed it in a metaphorical way, using his literature as a conduit to argue his own belief system and those of the people.