I would make sense for Zou to have a really important fruit, but Oda fumbled the bag
Or for the WG to have fed Zunisha an important DF so to keep it out of the reincarnation cycle, given how difficult it is to kill such a colossal elephant.
It would have been a bit of bitter-sweet irony to punish Zunisha for his crime (whatever that might have been) by making his existence tied to his captain's return: as long as Zunisha lives, JB can never be back, and Zunisha cannot willingly kill himself nor ask others to kill him.