Why do Naruto naruto have to die and Kai-kai doesn't die? The popular explanation is that the plot needs, Naruto at this stage has not yet obtained the six immortal chakras and special abilities, naturally can not save Ninji like saving Matkai, the following is detailed.
Naruto had not yet received the Six Immortal Chakra Bonus and did not have the conditions for Ninji when he died, and Naruto had successfully opened the Six Modes when he saved Matkai, and Ninji sacrificed too early to make Naruto powerless.
Naruto is not much of a means to truly resurrect people, except for chiyo's mother-in-law's own rebirth, the reincarnation of the samsaric eye is the special ability of yin and yang.
Unfortunately, the plot of Ninji's sacrifice takes place in episode 614 of the manga (episode 584 of the anime version), and although Naruto has reached a reconciliation with Nine Tails to become a perfect human pillar force and can also use the Immortal Mode, he does not have strong medical abilities and is powerless to face Ninji's death.
The reason why Naruto was able to resurrect Kai was because the previous six immortals had already given their own special chakra to Naruto so that he could use the special ability of the six levels of the six immortals Chakra to use the special ability of the six levels, including the ability to use the Yin and Yang to resurrect specific targets, and the time when Matkai launched the eight-door armor was already about level 644 in the anime version, Ning Ji had already received a box lunch, plus the six immortals Chakra Limited Naruto could not save the lives of all the ninja alliance members, in this case, Ninji's sacrifice was the price that had to be paid. War will definitely have someone sacrificed, some people can be saved by good luck, some people can't be saved if they are unlucky, and Ning Ci is a character with bad luck.
Ninji's sacrifice can be said to be that the plot killed Naruto, who had not yet reached his peak, so that Naruto could have a smooth finale.
Naruto's main plot is mainly Kuroji's plan to use the Infinite Moon Reading to revive the Night, and in order to lift the Infinite Moon Reading, Naruto and Sasuke are the key elements, so these two cannot die early.
The process of the Fourth Ninja War is not safe for Naruto and Sasuke even if there is a protagonist aura, Sasuke needless to say, before getting the power of the Six Immortals, he was almost solved by the Six Spots, if it were not for the fact that sasuke was saved, Naruto might have had to come to an end in advance, Naruto was also many adventures in turning the danger into a disaster, Ninji's role in the Fourth Ninja War was to receive a box lunch for Naruto.
Logically speaking, if Ninji did not do the cuttings for Naruto and Hinata, Naruto and Hinata would undoubtedly die, and once Naruto died, the Infinite Moon Reading would not be lifted, and the Night would not be sealed, so that Ninji would become a victim of the development of the plot.
Naruto was unable to fight against the Wooden Cutting Technique used by the Ten Tails before getting the Six Immortals Chakra, and due to the super fast release speed of the Ten Tails skill, the result was that Ninji had to give up using the Heavenly Sacrifice to give Naruto who could change the pattern of the ninja world to have a chance to survive, and when Naruto's upgrade was completed, Ninji's story mission was also over, and many people would wonder why Naruto, who had obtained the power of the Six Immortals, still had not revived Ninji.
Personally, I think the reason is that the number of six immortal special chakras is limited, Naruto's resurrection of Matekai and the reconstruction of Kakashi's eyes are already the limit, and it is also necessary to retain the six immortal chakras for sealing the Otoki Kiku night and lifting the Infinite Moon Reading.
The same problem is the same for Sasuke, who does not use the reincarnation eye to resurrect Ningji in order to prevent Sasuke from dying due to over-consumption of chakra.
Finally, I personally believe that the reason why Naruto did not use his special ability to save Ninji but to save Matekai was that Ninji died too early, and Naruto had not yet received the blessing of the Six Immortals, resulting in the inability to revive Ninji, plus the plot development required Ninji to become a victim of naruto's preservation. Please forgive me for the place where the analysis is not in place, and if you have other ideas, please leave a message to discuss.