Board Thread:Naruto Discussions/@comment-26403664-20160228225013/@comment-681696-20160314223146

JouXIII wrote: @Tasuxeda Except Kakashi also had Six Paths Chakra at the time.

And again, if Itachi can't even do full humanoid Susanoo(his best is armoured half-body Susanoo, after all), what chances he has for PS?

That is one theory about why Kakashi can use PS, but like my theory is just a theory not fact.

We don't know what Itachi's best Susanoo would be as we only see his susanoo while he is a edo tensei zombie and thus weaker than when he was alive and at the end of his battle with Sasuke after he used multiple high level techniques in a fight that he planned to lose.

Although having thought about it more I think my theory about how long Itachi could maintain PS was probably very wrong, I am now thinking that a fully rested Itachi would be suffering from chakra exhaustion before he even used PS for 30 seconds.