I just realized that I seldom write/read fanfiction of books, and when I do, it's because they've been adapted to multi-media. Like, when I was into the LoTR and Hobbit fandoms, my baseline was the movies. When I got into Hannibal, it was because of the TV show. Harry Potter--the recent video game. Even my most recent story, set in the Song of Ice and Fire, is based on the series.
The only time I've had the book-version in mind, was when I was looking for stories on Arya/Jaqen H'ghar.
Everything else, I imagined the assets and events used in the shows/movies.
It's the same with drawn media. Manga and comic books don't inspire the urge for fanfiction in me, but anime and movies do. I had no interest in reading Marvel fanfiction until the movies came out. I only got interested in fanfiction for Haikyuu, One Punch Man and Chainsaw Man when I saw the anime.
The type of media I tend to write for is video games and the ones I read from are TV shows. I have no idea why, but I just thought it was interesting how certain media can make my brain latch onto the IP more creatively than others.
How about you guys? Any media preferences as well or maybe you have other factors in play?