One possible reason could be that Naruto has a stronger willpower in the fanfic. His never - give - up attitude might be enhanced, allowing him to push past his limits more easily than Sasuke. For example, he might have an even deeper connection to the Nine - Tails' power, enabling him to draw on it more effectively in battles.
In some fanfics, Naruto's character development might be focused on his physical and mental growth in a way that surpasses Sasuke's. He could be depicted as having a more natural affinity for chakra control and a greater understanding of the nature of chakra, which makes him stronger when it comes to combat and other ninja skills.
Maybe in these fanfics, Naruto has received some unique training that Sasuke didn't. This special training could have unlocked new abilities or enhanced his existing ones. Like he could have trained with a hermit who taught him some ancient and powerful jutsu that gives him an edge over Sasuke.
It could also be that the writer wants to subvert the traditional power dynamics. In the original series, Sasuke is often seen as very powerful. But in this fanfiction, Sakura might be stronger because she has a stronger willpower. She could be more determined to protect her loved ones and this gives her an extra boost in strength. For example, her love for the village and her friends drives her to train harder and unlock new levels of power.
One possible reason could be that Naruto has developed some new and extremely powerful jutsu in the fanfiction. Maybe he has found a way to combine his Sage Mode with his Kurama chakra in an even more efficient and powerful way that gives him an edge over Madara.
One possible reason could be that Naruto has access to some new and powerful jutsu. Maybe he discovers a secret family technique or is taught by a very powerful sage. This new knowledge gives him an edge over Kakashi.
One reason is that Naruto has the Nine - Tails' chakra. In fanfic, this power can be further enhanced or manipulated in ways that make him far more powerful than Sasuke. Another reason could be his more outgoing personality. He might be able to form alliances more easily, leading to him having more resources and power - ups at his disposal.
Another plotline might be Naruto's strength leading to a different kind of peace in the ninja world. Since he's stronger, he can enforce his ideals more effectively. He could start a new era of cooperation among the ninja villages by using his power to make the other villages see the benefits of unity, and Sasuke might be reluctantly following his lead.
One reason could be Naruto's unwavering determination. He has always been able to push past his limits. In many battles, his willpower has been a key factor in his victory. Another could be his unique chakra nature and the amount of chakra he has at his disposal. Naruto has a vast reserve of chakra, which gives him an edge in long - drawn battles. Also, his ability to form strong bonds with others might contribute. These bonds could potentially unlock new powers or give him the strength to fight against Kaguya.
Well, Naruto has had the advantage of learning from many great shinobi over time. He also has a unique bond with his friends which could potentially give him more strength. Minato, on the other hand, had a shorter lifespan to develop his powers. Naruto has had more time to grow and surpass his father in strength.
One reason could be Sasuke's thirst for power. If he believes Naruto is the only obstacle in his path to ultimate power, he might be driven to kill him. In some fanfictions, Sasuke may be influenced by a dark entity that twists his mind into thinking Naruto is a threat that needs to be eliminated.
It could also be due to a misunderstanding. Perhaps there is some misinformation spread about Sasuke in the fanfiction, like false rumors that he has a hidden and malevolent plan. Naruto, being someone who values their friendship deeply, would be scared at the thought of Sasuke turning into an enemy because of these untrue stories.
In a lot of fanfics, Naruto is stronger than Sasuke. Naruto has always had an incredible willpower and in fanfics, this can be translated into him achieving greater power levels. His ability to make friends and allies can also lead to him getting more support and resources in the story. For example, he might get some special training from an ancient sage that Sasuke doesn't have access to. Also, his positive attitude can attract more powerful beings to help him out, which gives him an edge over Sasuke in terms of strength.