Board Thread:Naruto Discussions/@comment-3568954-20140806112253/@comment-24531362-20140809024249

Obito never used Susanoo because he only had both eyes and consciousness for the duration of time that he used them for the interdimensional Kamui shopping spree with Sakura. He could have used it, but never did.

As for the Perfect Susanoo, I think that either

A) Kakashi just focused really hard on attaining the Susanoo, armed with knowledge of its existence (something Sasuke and, presumably, Madara lacked) and Naruto is an idiot who doesn't understand the Sharingan.

B) Obito used his gift chakra to accelerate the unlocking process, because he's already transcendent over planes of existence and who knows what else at this point, and Naruto is an idiot who doesn't understand the Sharingan.

C) Kakashi did actually copy the Perfect Susanoo from Sasuke in the split second that he had both eyes, because there is no reason that Sharingan users cannot copy Sharingan techniques which they would be able to use naturally at some point (i.e. Susanoo but not Amaterasu). Naruto is still an idiot, but he does understand the Sharingan.