Because Zoro wasn't the one who turned Shusui into a Black Blade was he ever able to use it to its full potential?
We know that unlike Enma Zoro had an easier time learning to control Shusui because of the fact Shusui accepted him as worthy after he defeated the "Body" of Ryuma.
But do you think Zoro ever actually mastered Shusui to the level of Ryuma? Or could Shusui still have had more that it have done if used by Ryuma its true master?