Board Thread:Theories and Speculation/@comment-26904721-20150820181613/@comment-24949460-20150822224228

The reason MN is a Senju is because of the way he has an ability that only those of connected to the Ootsutsuki possess. Whether you believe it or not is not up for debate. The fact of the matter is the moment it was found out that chakra mode is not a tailed beast skill but a skill of the Ootsutsuki bloodline. And the only confirmed bloodline that is a part of Konoha(Minato birthplace) are the Uchiha and the Senju. Plus Kishimoto did kind of state it in the manga. But in a way people wouldn't get it at first since there is a lot of other things going on that prevented people from focusing on that open fact.