Well, his character is in a high - stakes environment, a hospital. Fanfic about him can explore all kinds of medical dramas and ethical dilemmas. Also, his good looks and his growth throughout the series make him an appealing subject for fanfic. Writers can create different storylines around his development, both as a doctor and as a person. For example, they can write about his love life, his rivalries, and his friendships. There are so many angles to take when it comes to writing about Jackson Avery.
The popularity of Jackson Avery fanfic also lies in the fact that the show he's from has a large and dedicated fan base. Fans are already invested in his character and want to see more of him. Fanfic allows them to see different versions of his story, whether it's him taking on a new specialty in medicine, or him getting into a new relationship. It gives the fans a chance to explore 'what if' scenarios and that's really appealing.
Jackson Avery is an interesting character. His fanfic is popular because he has a complex personality. He's talented, ambitious, and has a certain charm. Fanfic writers like to explore his character further, like his relationships with colleagues and patients.