Board Thread:Naruto Discussions/@comment-30168130-20170706050832/@comment-6769189-20170710005124

Its about speed/sneakyness. Thats one of the reasons substitution isn't typically used against a faster/more focused opponent.

We have seen Itachi at one point fast enough to cast hand signs for a jutsu and draw shuriken without Kakashi and his Sharingan noticing the hand signs.

We can also add that substitution is rendered moot by most of the higher tier ninja because it is little more than a parlor trick that against their speed/perceptiveness.