Another reason might be that they faced some sort of writer's block. Coming up with new plot ideas, especially for a crossover as complex as Naruto and Devil May Cry, can be really challenging. They might have hit a point where they didn't know how to progress the story further. Or perhaps they received some negative feedback early on that discouraged them from continuing.
Well, it could have been about a character from Naruto ending up in the Devil May Cry universe. Maybe Naruto himself gets transported there and has to use his ninja skills to fight demons. He might learn new abilities from the Devil May Cry characters, like Dante's swordsmanship or Vergil's cool moves. And perhaps there would be some sort of conflict between the ninja way and the demon - hunting way of the Devil May Cry world.
The Naruto world has a rich lore with tailed beasts and different bloodlines. The Devil May Cry world has its own lore regarding demons and the underworld. In fanfictions that combine these two, the merging of these lores can result in really unique storylines. For example, a fanfiction could explore what would happen if a tailed beast was possessed by a powerful demon from the Devil May Cry world, and how the characters from both universes would deal with such a situation.
One possible reason for a 'Naruto abandoned fanfiction' could be the author losing interest. Maybe they started a new project or real - life got in the way. For example, a fanfiction that was exploring an alternate ending where Naruto doesn't become Hokage. The author might have abandoned it because they couldn't figure out how to develop the complex relationships between the characters further. Another work could be one that was delving into the backstories of minor characters but was dropped due to lack of readership feedback.
Well, in a fanfiction like this, maybe Naruto is given the devil fruit as a gift from a mysterious being. However, he quickly realizes that it's affecting his chakra system in a negative way. So, he abandons it. Later, an enemy gets hold of the fruit and becomes extremely powerful. Naruto then has to use his ninja skills and the power of friendship to defeat this new foe.
Naruto's ability to create shadow clones would be very useful. He could send clones to scout out areas full of demons, or use them as distractions in battle. His rasengan could be a powerful close - range attack against demons. Also, his chakra control could potentially be used to enhance Devil May Cry weapons, like making a sword cut through tougher demon hides more easily.
Another reason is that the writer may lose their passion for Naruto over time. They might have been really into the series when they started the fanfiction, but as they move on to other interests or the Naruto series itself changes in a way they don't like, they abandon their fanfiction. For instance, if a new arc in the Naruto anime or manga goes in a direction that the writer doesn't enjoy, it could make them less interested in writing their own story within that universe.
One great Naruto Kushina Devil May Cry fanfiction could be about Kushina having some sort of connection to the Devil May Cry world. Maybe she has a hidden power that is similar to the devil arms in Devil May Cry. For example, her chakra chains could be related to a special weapon in the Devil May Cry universe.
Perhaps it could be a new character created specifically for the fanfiction. A young ninja from a minor clan who showed extraordinary talent in a unique form of jutsu. But because their clan was small and not very influential, and the larger clans felt threatened by this new talent, they managed to get the village to abandon this prodigy.
Yamato's wood - style jutsu could be seen as a form of nature - based magic in the Devil May Cry world. He could use it to create barriers against demons or even trap them, much like how Dante uses his devil arms to fight. His ability to sense chakra might also translate to sensing demonic energy, allowing him to detect hidden demons.
The ninja world could change a great deal. If Naruto abandons a devil fruit, and it falls into the wrong hands, like say Orochimaru's, he could use it to create even more powerful and terrifying jutsu. This would force the other ninja villages to band together against this new threat. Also, the concept of power in the Naruto world would be redefined as devil fruit powers are so different from chakra - based abilities.