The novel is based off and around the character of Dracula, yet we only see glimpses of him in the narrative, why did Stoker do this?
Throughout the plot we meet him in the begining, then we don't see him very often, only through brief occurances, newspapers, psychic phenomena, ect. Why would Stoker write him in like this opposed to giving more insight into the character of Dracula himself?
The elusiveness of Dracula lends to the suspense and angst of the story. As the master of darkness, he hides in the shadows and cannot be approached, but he can appear of his own will at any moment. The anticipation of his presence is as bad or worse that the thing itself.
However, this 'stalling' technique in epistolary form has often been criticized as the weakness of the novel. Indeed, all the soul-searching and speculation over the struggle between good and evil today would seem obfuscaory, even obsolete. One must remember the social context under which Stoker wrote 'Dracula,' a provocative work in which he raised some serious questions concerning sexuality, particular for women.
I agree with parkerlee's answer above, but here is another aspect of this approach. Dracula is presented to us as a character so far outside the experience of the "normal" human that we cannot enter into his mind, as we would in a normal narrative. Where we as readers might be presented with insight into a regular literary character through references to what he thinks and does, we are only presented with Dracula through the eyes of others. Those others cannot see his actions behind the scenes, nor can they truly understand his thoughts and the motivations of his aims. His true background and aims are only faintly understood by us and the other characters of the novel, and the processes of his mind which guide his choices and actions are a mystery normal humans cannot fathom. He is a truly alien creature who only looks like a man. The Victorian audience for whom the book was written was as profoundly disturbed by the distance between Dracula and knowable human emotions and motivations as they were by the sexuality and perverseness of the relationship between Dracula and his victims.