One possible reason could be a powerful genjutsu cast over the village. Maybe an enemy ninja with strong illusionary powers managed to make everyone forget Naruto. It could also be due to some sort of curse. For example, a dark curse was placed on Naruto that made people's memories of him fade away gradually.
Well, perhaps there was some kind of time - related magic or jutsu involved. Imagine a jutsu that distorted time in a way that Naruto's existence was erased from people's memories. Another reason could be that Naruto sacrificed himself in a way that required everyone to forget him to protect the village. His selfless act might have come with the price of being forgotten.
One possible reason could be that Naruto has seen or experienced something that makes him distrust or fear those around him. Maybe he has discovered a dark secret about the village or his friends that has shaken his beliefs.
It could also be that Naruto has had some sort of revelation about himself or the ninja world. He might have realized that the relationships he has been in were holding him back in some way. Maybe he wants to focus on self - improvement and growth, and he believes that being alone is the best way to do that. He could be trying to find his true self without the influence of others.
Maybe he got some kind of curse. In the Naruto universe, there are all sorts of dark magic and curses. If he was cursed, it could make him withdrawn and cause him to ignore everyone.
It could be due to a personal loss that has hit him really hard. Maybe he lost someone very close to him, like Jiraiya all over again, but this time it has broken him in a way that he shuts everyone out. He might also be under the influence of a new power that he doesn't fully understand yet, and this power is affecting his emotions, making him cold as a side - effect until he learns to control it.
One reason could be to explore a darker side of the Naruto world. Writers might want to see how Naruto's character would develop under extreme circumstances. Another reason could be to create a contrast. By showing Naruto being abused by everyone, it makes the positive aspects of his relationships in the original series seem even more significant.
Jealousy could also play a part. Naruto becomes very powerful over time. Some of his peers or even superiors in the village might be jealous of his growth and popularity. They could spread lies and manipulate others into believing that Naruto is a threat. This would gradually turn the whole village against him. For example, a less - known ninja who feels overshadowed by Naruto's achievements might start the ball rolling on the betrayal.
One possible reason could be a false accusation. Maybe someone spread a rumor that Naruto was involved in a heinous act, and without proper investigation, everyone turned against him.
One possible reason could be that Naruto has had some really bad experiences that made him lose his confidence. For example, if he faced a series of powerful enemies alone and got severely beaten each time, it might make him scared of everyone. Another reason could be some kind of curse or dark magic that affects his mind, making him fear those around him.
Perhaps in the fanfiction, Dark Naruto has been betrayed. It could be that someone he trusted deeply, like a mentor or a close friend, turned on him. This act of betrayal could make him lose faith in humanity and lead to his hatred for everyone. He might think that if one person can betray him, then anyone can, so he hates them all without distinction.
One reason could be that he's under a lot of stress. Maybe he's facing a powerful enemy and the pressure is getting to him. So he snaps at everyone without realizing it.