Perhaps his parents are in a coma or some other incapacitated state. Maybe they got injured during a major battle and are being kept alive but unconscious. So, they are not in a position to notice Naruto. Or it could be that there is a memory - wiping jutsu that has been used on them, causing them to forget they have a son. This could be the result of a political conspiracy within the village.
One possible reason could be that his parents are extremely busy with their own important ninja duties. In the Naruto universe, ninjas often have to go on dangerous missions. His parents might be so preoccupied with protecting the village from external threats that they simply don't have the time to notice Naruto. Another reason could be some sort of curse or spell that has made them oblivious to Naruto's existence. This could be part of an evil plot by an enemy who wants to isolate Naruto and make him feel alone.
One possible reason could be a curse. Maybe some enemy put a curse on Naruto's parents that made them forget about him. It could be a way to get back at them by targeting their most precious thing, which is their son.
He could hate them because he felt they were absent during his difficult childhood. Naruto faced a lot of loneliness and bullying, and if he thought his parents could have prevented it but didn't, that would breed hatred.
Maybe they were under some kind of curse or dark magic that made them fear they would harm Naruto if they were close to him.
One possible reason could be that Naruto was taken for his special chakra. In the Naruto universe, his chakra is unique as he is the jinchuriki of the Nine - Tails. Some evil forces might want to control or study his chakra, so they take him from his parents. Another reason could be related to a prophecy. Maybe there are those who believe Naruto will bring great change, either good or bad according to their perspective, and they take him to try and influence his future.
One possible reason could be that they are too involved in their own shinobi duties. The world of ninjas is full of dangerous missions, and they might be constantly away on important tasks, leaving Naruto feeling neglected.
Naruto would likely become more independent. Without his parents' attention, he would have to fend for himself more. He might grow up quicker and be more self - reliant. For example, he would have to learn to cook and clean for himself at a much younger age.
One possible reason could be some past misdeeds by the parents. Maybe they were neglectful or made decisions that seriously affected Ciel's life in a negative way in the context of the fanfic.
One possible reason could be that Naruto has some sort of power within him that they fear. Maybe it's a power that they don't understand and think could be dangerous, so they hate him out of fear.
Poison or some sort of harmful jutsu effect. Maybe an enemy uses a special poison or a jutsu that slowly weakens Naruto's body. In the fanfic, this could be a new and unique type of attack that he has not encountered before, and his body is not able to withstand it, resulting in fainting.
Maybe they were forced into it by some dark magic or a powerful dark wizard. They could have been blackmailed into betraying Harry to save their own lives or the lives of others they cared about.