The allure of the unknown. Vampires are often mysterious in anime, and when they bite a reader - insert character, it creates a sense of danger and excitement. It's like the reader is being drawn into a world that is both dark and alluring.
It plays on the idea of a forbidden connection. In most stories, vampires are not supposed to interact with humans in such an intimate way as biting. So when it happens in fanfiction, it's a new and exciting take on the relationship. It also allows for a lot of character development, as the reader - insert can go through a transformation or gain new insights after the bite. For example, they might start to see the vampire world from a different perspective or develop special abilities.
The idea of a vampire bite in anime fanfiction is also popular because it can be a very emotional and physical experience for the reader - insert. It can be a moment of great pain, but also of pleasure in some cases. This duality makes it interesting to explore. Additionally, it can be used as a plot device to drive the story forward, whether it's starting a love - hate relationship between the reader - insert and the vampire or uncovering some long - hidden secrets about the vampire or the world they live in.