What is the mastery difficulty for Kha'Zix? I can't seem to get anything above A!

I've played a lot of Kha'Zix (I love the guy) but I don't believe I've ever received a single S Rank with him. Here's an image from a recent game I got out of.

Does Kha'Zix require something special as opposed to other Champions to get an S Rank? I primarily play him Top and Mid. In this specific game I was going Mid lane.

EDIT: Could it be my cs score? I'm bronze, so my cs isn't up to par. Is that the only reason?