Board Thread:Theories and Speculation/@comment-31477926-20170702121758/@comment-7292165-20170819202222

He used Susanoo only to protect Sasuke while the technique was activated. Why not use Susanoo and in turn the shield to protect him and himself before Kabuto even reached him.

On a side note, Sasuke Susanoo did deactivate, but it put him in that much pain, but Itachi didn't seem to be in such pain. He could still pull off using Susanoo and still concentrate while Sasuke could not.