This is some good shit right here- No S-,S, OR S+

Typically I would say that cs was your problem, but you were playing support. I got nothing. Wards placed maybe?

You had good wards. Good Kill Participation (above 50%), no deaths. My next guesses are objective participation. How much damage did you do? etc etc. It seems like you're very borderline to have gotten an S.

Sadly in games past the 20 min mark CS ranking is really harsh, even when playing support. Kinda dumb that it works like that, but oh well.

Towers and possibly gold earned since you sold your supp item. You probably had little map presence? I've gotten a lot of S's on Zilean. And it's usually because I frequently drop deep wards in enemy territory, or roam to mid. Even better? Hope you get zilean on ARAM. Absolutely easy to get S's there: went 12/0/30. Bought Tear, boots, and a health pot. Still had a health pot left at game end.

The system scores you primarily based on your stats compared to other who have played the champion. So, there's something that you lacked that other zileans were doing better.