Well, it gives fans a chance to explore uncharted territory in the Supernatural universe. The relationship between Castiel and Dean has always been complex and fascinating. By making Castiel female, it adds another layer of complexity. It can also attract a wider audience, including those who might be more interested in stories with female leads or different gender dynamics in relationships.
The popularity also comes from the fact that fans love to imagine different versions of their favorite characters. A fem Castiel brings a whole new set of possibilities in terms of character development and relationship building with Dean. It allows for stories that might not be possible with the original male Castiel. For example, in some fanfictions, fem Castiel might show more vulnerability or a different kind of strength, which makes for really interesting reading when paired with Dean's character.