From what we have been given and seen in the story information wise, the main reason is because if Naruto dies, Kurama can not be reincarnated properly.
As we all know from what Raikage said, if he were to kill Naruto Uzumaki, Kurama would reincarnate after a bit of time. But what most people do not know in the Naruto universe is that Naruto only had half of Kurama’s chakra.
The yang half, or the life half. All 1-9 tailed beasts are composed of two chakra types. Yin and Yang. Yang is for life force and Yin is for bringing form to that life force. So Kurama in Naruto was missing the Yin chakra.