kaiserin-erzsebet:

It isn’t explained in the text, so you aren’t missing anything.

So, as with most things in the book, there is a Doylist and Watsonian reading. The Doylist one is rather boring, so I’ll start with that. If the Count moved on and preyed on someone else, we would have to shift to some other set of characters somewhere else in London. Which would understandably make the narrative bigger. So Stoker chooses to keep it limited.

But I really do think the Watsonian explanation is way better in this case. He won’t give up because he has laid claim to someone and he refuses to be defeated or told no. She isn’t particularly special to him- she’s just the person who sleepwalked into the graveyard. But like with his claim on Jonathan (“this man is mine”), once he has decided to sink his teeth into someone, they become his. He’ll come back to Lucy over and over because he’s claimed her.

The fact that someone is clearly giving her blood and guarding her room against him means that there is a battle being fought for her soul. He knows he has adversaries. So no matter how much easier it would be to find someone else, he will not let anyone keep him from what he thinks is his. He will do whatever he has to in order to get past those guards, because that’s how he proves that no one can stop his claim.

And he’ll also leave her with just enough life that the people trying to thwart him have to watch her fade away, because that’s how he shows that he’s more powerful than them. It isn’t about Lucy, it’s about winning.