It's all about the contrast. Sailor Moon is about light, love, and friendship. Vampires, on the other hand, are often associated with darkness, death, and mystery. When you put these two together in fanfiction, it creates a really interesting dynamic. Writers can play with the idea of how Sailor Moon and her friends would react to the presence of vampires. They can also explore how vampires might be affected by the positive energy of the Sailor Scouts. Plus, it gives a chance to create new and exciting storylines that deviate from the original Sailor Moon plot.
One major factor is the potential for new relationships. In Sailor Moon vampire fanfiction, you can have romantic relationships between the Sailor Scouts and vampires, which is a big draw for fans. It also allows for new character development. The Scouts might have to overcome their prejudices about vampires, and vampires might have to change their ways in the presence of the Scouts' positive influence. Additionally, the fanfiction can explore different power dynamics. Vampires have their own set of powers, and seeing how they interact with the Sailor Scouts' powers is really exciting for readers.