Board Thread:Naruto Discussions/@comment-33026485-20170904034338/@comment-5085906-20170904105734

It was not author oversight nor was it done solely for political reasons, albeit there are politics involved in it mainly Konoha's good relations with Uzushiogakure, the Uzumaki's village.

In Chapter 500 of the manga, Kushina said and I quote: "I was born with very strong chakra, even for Uzushio, strong enough to control the Kyubi. My predecessor was also a kunoichi from Uzushio so it was custom."

And while Tsunade's chakra reserves are probably large enough to contain Kurama, she doesn't even half of Kushina's aptitude with advanced sealing jutsu that could keep him at bay.