The characters themselves are very interesting. Fans like to explore different scenarios for them. In fanfic, we can see alternative universes or different points in their relationship. For example, some fanfics might show how they first met in a completely different way. Also, the chemistry between them in the original story is so strong that it gives a lot of inspiration for fanfic writers to expand on their story.
The appeal also lies in the fact that the original story leaves a lot of room for interpretation. Fans want to fill in the gaps and explore the unsaid aspects of Eve and Villanelle's relationship. Fanfic allows them to do that. It can be a way to further analyze their personalities, their motives, and how they might react in various situations that the original show didn't cover. This freedom to create and explore different ideas is what makes eve and villanelle fanfic so popular among fans.
The complex relationship between Eve and Villanelle makes the fanfic appealing. Their love - hate dynamic is full of tension and mystery.