It was because of the tradition that shogun's lawful wife must be a daughter from Gosekke (five families of Fujiwara clan) or from Miyake (families allowed to have the status of Imperial family) and it was unprecedented for a daughter of daimyo (Japanese feudal lord) especially tozama daimyo (non-Tokugawa daimyo) to become shogun's lawful wife.
将軍家の正室は五摂家か宮家の姫というのが慣例で、大名の娘、しかも外様大名の姫というのは全く前例がなかったからである。