Board Thread:Versus Debates/@comment-5936871-20160126112547/@comment-5936871-20160131031138

Because Shisui would have had a regular susanoo anyway simply due to already unlocking the power of both of his MS in the actual canon, it just wasn't shown. Itachi on the other hand, never did have a perfect susanoo at any point in the canon whatsoever and was incapable of using it. Itachi's PS is completely non-canon, Shisui's susanoo is something he would have already had but it just wasn't shown.