Board Thread:Theories and Speculation/@comment-26422608-20150522042352/@comment-25549954-20150522051929

Because they are just descendants and only the reincarnations are able to awaken it, Asura chose Naruto's body and Indra chose Sasuke's body and by combining the DNA they are able to awaken trait of Hagoromo, the Rinnegan. Nobody would awaken it other than reincarnations not even Obito, Danzo- half of their body was made of Hashirama's cells and they had Sharingan yet they couldn't awaken it.