One possible reason could be that J.K. Rowling, the author of Harry Potter, has strict control over the franchise and might have made some regulations that made it difficult for him to continue. Another reason could be that he simply lost interest in writing fanfiction about it. Maybe he wanted to focus on creating his own original stories instead.
Perhaps he faced a lot of criticism from the fan community or even legal issues related to fanfiction. Fanfiction sometimes toes the line of copyright infringement, and if he got into trouble because of it, he might have decided to give it up. Also, he could have grown out of the phase of writing fan - made stories about Harry Potter and moved on to other hobbies or writing interests.
One possible reason could be that Ginny realizes Harry is too preoccupied with his heroic duties and she feels neglected. In fanfiction, this is a common trope where the female character desires more attention in a relationship.
One possible reason could be that he realizes his feelings are unrequited. In the fanfiction, perhaps Lily has made it clear she doesn't have the same feelings for James, so he gives up. Another reason might be external factors. For example, there could be some sort of danger or situation that makes James think he needs to step back from Lily for her own good.
It could also be that he is tired of the false expectations created by fanfiction. In fanfiction, his character might be portrayed in ways that don't match who he really is. He wants to be true to himself and his ninja way, so he gives up fanfiction.
One possible reason could be a lack of time. Maybe Ash has become too busy with other aspects of life, like work or studies, and can't find the time to write fanfiction anymore.
Perhaps he simply lost his creative inspiration. Fanfiction writing often requires a constant flow of new ideas, and if he's been through a lot of stress or has had some personal issues, like his complex relationship with Lily still haunting him, it could dry up his well of creativity.
One possible reason could be that Izuku realizes Bakugou's unchanging and hostile attitude. Despite his efforts to be friends or get along, if Bakugou continuously rejects him or is overly aggressive, Izuku might feel it's a lost cause.
One possible reason could be that she loses inspiration. Maybe she has explored all the ideas she initially had for her fanfiction and can't come up with new and exciting storylines. Another reason might be lack of time. If she gets involved in more important or urgent matters in the RWBY universe, she simply doesn't have the hours to sit down and write fanfiction anymore. Also, she could face criticism from others which makes her lose confidence in her writing and decide to give it up.
It could be that in this fanfiction, Ash had some personal issues come up. For example, he might have received some bad news from his family back home that made him lose focus on training and eventually give it up. Another reason could be that he found a new passion or interest that diverted his attention away from training.
Maybe Stiles has been constantly disappointed by Derek's actions or lack of response in the fanfiction. It could be that Derek has repeatedly let Stiles down, and over time, Stiles just can't take it anymore.
One reason could be repeated heartbreaks. If she has been through multiple bad relationships with men in the fanfiction, it would lead her to give up. Maybe she constantly faced betrayal or being taken for granted.