How is this not a S?

Grades are based on more than just KDA. This may include things like damage/healing, CS, objectives etc.

Look at it like your scores in those are each given a grade, then averaged. You might get an S+ KDA, but poor scores in other areas drag it down.

I see...

Your cs, you have 1/2 of the farm you're supposed to have, with just 50 more minions you would have got your precious S, now, if you need it for the chest, do as i did when i had troll retards and they cucked my S rank, i played another game hoping to not have retards, but that's a riot's problem, not mine. Also nice skype dude. And internet explorer in the toolbar. Tell that twitch to stop building like that, going bork into runaan should be better. And tell kha he's not funny, ER on kha is simply retarded.

You're graded on how well you play with a champ relative to other players of the same champ of similar MMR. Not getting an S means you simply didn't play as good as others did, overall.

Your CS is terrible for the game length. 40+ minutes and only 200 cs is not that good.