Board Thread:Theories and Speculation/@comment-25547449-20160805204709/@comment-4912646-20160806012622

I think this theory is probably correct. It was never really explained what exactly is needed for the Ten-Tails to turn back into Kaguya, but we know Black Zetsu initiated the process and a massive amount of chakra was required. Kaguya probably wouldn't have turned into Ten-Tails in the first place if she wouldn't be able to turn back, so her chakra being spread around the world would go a long way to explain why she couldn't just turn back the instant Obito revived Ten-Tails and it fully matured.

I really hate how the anime messed up the Kaguya/Ten-Tails stuff so badly, because now we'll probably never know why she bothered turning into a rampaging monster in the first place.