The world of Highschool DxD itself is full of magic, supernatural elements, and complex relationships. When combined with yuri themes involving Rossweisse, it creates a new and exciting dynamic. Writers can play with the existing lore and create scenarios where Rossweisse's relationships with other female characters are developed in ways that are different from the original story. This allows for a lot of creativity and exploration of different relationship dynamics.
The unique character of Rossweisse in the Highschool DxD universe is one factor. Her backstory and personality traits can be interestingly explored in a yuri context. For example, her role as a Valkyrie and her interactions with other female characters can be re - imagined in a romantic or friendly way that yuri fanfictions often do.
Yuri fanfictions in general have a charm of their own. They offer a different perspective on relationships, often focusing on the emotional and psychological aspects of female - female relationships. In the case of Highschool DxD Rossweisse yuri fanfiction, fans are attracted to seeing their favorite character in a different light, in relationships that may not be explored in the main series. It also gives an opportunity to see more female - centered storylines in a male - dominated anime like Highschool DxD.