Maybe the lack of reaction in Harry Potter fanfiction is due to a lack of promotion. There are so many fanfictions out there that if an author doesn't share it widely on relevant platforms or communities, it may go unnoticed. Additionally, some fanfictions might be too niche in their themes. For example, if it focuses on a very minor character's backstory that most fans aren't interested in, it won't get much of a reaction.
Sure, 13 Reasons Why is classified as fiction. It's not based on real events but rather crafted by the author's imagination to tell a compelling story and raise important discussions.
The main reason is that fans are passionate and want to add their own twists and turns. Maybe they want to give characters new relationships or adventures. It's a way for them to be part of the story and have fun with it.
Maybe it's because people are interested in exploring different relationship dynamics within the show's context. Some fans might find the characters' personalities and interactions inspiring for such creative works.
There could be a few reasons. One could be that fans are really passionate about the show and want to take it in a more mature or risque direction. Another reason might be that they think it adds an extra layer of drama or excitement to the original story.