Well, it could be that Snape framed Harry for something serious, causing him to be expelled and leave Hogwarts. Maybe it was a misunderstanding or a plot by Snape to get rid of Harry for some reason related to their past conflicts.
One possibility is that Snape discovered Harry was involved in a forbidden magic practice and insisted on his leaving. Or perhaps Snape had a personal vendetta against Harry and used his authority to force him out of the school.
In some fanfictions, it might be because Snape exposed a secret of Harry's that led to his departure.
It could also be that Snape represents everything that Harry hates about Hogwarts at that moment in the fanfiction. Snape's strictness, his partiality towards certain students, and his general unfriendly attitude might make Harry feel that Hogwarts is no longer the safe haven it once was. So, he leaves, thinking that he can find a better place where he doesn't have to deal with Snape's negative presence.
One possible reason in fanfiction could be that Snape's constant bullying and unfair treatment finally pushed Harry over the edge. Harry might have felt that he couldn't take it anymore, especially if Snape made his life at Hogwarts unbearable in ways that the other teachers didn't notice or couldn't stop.
One possible reason could be that he feels he has fulfilled his purpose there. Maybe he has achieved certain goals related to protecting Harry Potter secretly, and he decides it's time to move on to other things. For example, in some fanfictions, he might leave to explore his own family history or to further his research on the Dark Arts in a more secluded place.
In fanfiction, one reason could be his desire for a fresh start away from the memories at Hogwarts. Maybe he feels he has done all he can there and wants to explore new magical knowledge elsewhere.
He might leave Hogwarts in fanfic due to a sense of disillusionment. Maybe he discovers some dark secrets about the school or the wizarding world in general that make him question his place there. It could be something related to the way the school is run or a conspiracy among the teachers that he stumbles upon.
Perhaps in this fanfiction, Snape has proven himself to Harry in some extraordinary way. Maybe he has saved Harry's life multiple times or has shown his true loyalties that were hidden in the original story. It could also be that Snape has some special connection with Harry that makes Harry trust him unconditionally.
Perhaps it's because Hogwarts is the central and most iconic location in the Harry Potter universe. Fanfiction often revolves around the events and characters associated with Hogwarts. So, it gives the impression that Hogwarts is like an ever - present entity 'watching' over all the different fan - created stories.
It's popular because Snape is such a complex character. His relationship with Harry in the original series is full of tension and mystery. Fanfiction allows writers to explore different aspects of their relationship, like what if they had been friends or if Snape had been more open with Harry from the start.
One reason could be that Harry Potter is the main character of the series. His story arc is central, and Snape is often a complex figure in relation to Harry. Writers might be more interested in exploring how Harry's actions and growth impact Snape's character and vice versa. Also, Harry has a broader range of relationships within the wizarding world that can be intertwined with Snape's story in fanfiction.
There are 'Harry Potter closed Hogwarts' fanfictions because it offers a new and interesting scenario. It allows writers to explore different aspects of the Hogwarts world that we don't usually see. For example, how the characters interact when they are cut off from the outside world.