Orochimaru is part Zetsu, so he will be the first to become a white Zetsu because all of his chakra was swiped by Kaguya. In other words there is nothing left to keep the Zetsu from taking over
Orochimaru was defeated by Itachi and then by Sasuke both descendants of Kaguya if Her great grandchildren can defeat Him, why not the one who started it all?