Not even an S- ?

You have a really good kda, but your cs is pretty bad which is one reason your score is lower

need high CS to get an S, ive had shitty score lines but had cs far above anyone else in a game and got an S

Sorry you didn't sit in the jungle all game and farm so Riot doesn't want to give you and S-

Its based on stats of other players too. I am betting down in the depths of bronze some eve's get 20+ kills and 150+ CS.