No s+ for 28/1 with 211 cs at 26 min?

okay, so i just got a new mouse after using a laggy wireless one for 3 days, so i decided to play my main, yasuo with this new mouse. i went 28/1, got 2 quadra kills, and 211 cs at 26 minutes, and at the end of the game i get an S? I killed 6 of the 10 towers killed that game, and used my ward every time i could, and i got both dragons. Granted, i did only get 4 assists, but when im carrying that hard, i dont think it should really matter. anyways, this ranking system is messed up.