The explanation would be that Oda wanted to give a "consolation prize" to Carrot fans... the problem is that it had zero build-up and the way it is done is awful.
Like, people complaint that Yamato "suddenly" wanting to protect Wano instead of joining makes no sense... but at least there was some foreshadowing about that decision, like Yamato having a guardian's role (swearing to protect Momo even if she has to die for it, telling Kaido that she can't call herself Oden if she doesn't defend the country, having the Wano's guardian deity devil fruit).
But Carrot as Zou's ruler? A completely nonsensical decision that Oda asspulled out of his ass, the reason being the stupidest in the series. (damn she visited TWO islands!!! this certainly makes this carefree 15 yo girl without experience much more fit to lead than anybody else!)