Okay, what the F*k am I doing wrong

you had less than 5 cspm

YOU DIED

What was your cs score? And how long was the game for?

Lots of factors go into S grades - I've found out the hard way too bro 4 A's in a row with 15+ kills at one point

Deaths. Cs. Assists.

Pretty sure someone on your team did good too and S rank is based on how well your team is doing too, to prevent abuse of the system. Otherwise people could try to get on at the same time in pre-mades and try to get S ranks that way by throwing games and etc. I mean your performance relies on your team mates doing well as well. So if they did good then why would you get a higher rank? At least that's what I suspect. I went like 18/3/33 on Sona a long time ago and got an A+ and the only reasoning I could find from what I saw was the fellow team mates did good/better than myself.

Going to presume it is that 15/0/7 Jhin. Who participated in in a total of 22 kills, but had a 7 spread with team mates while you on the other hand have 35 participation kills and only 3 spread out. I'm not saying they are penalizing you for your team playing better, but because your team mate is playing well could contribute to your lack of S because what they do ultimately also effects you.