Legit question: How was this not an S rank?

My last game was one of the best games ive had in....forever. Ive been in the absolute pits lately but i finally refreshed myself and had a good performance. One I thought was worthy of an S rank, and it somehow wasnt? 8/0/12, 200+ CS, not sure how many wards, and 17k+ damage, i think 3 or 4 towers i last hit myself. But that wasn't an S rank? Can someone explain to me what i was missing or lacking? I was SO happy like legitimately beaming because i finally did well and won a game i actually deserved to win and i was hoping i would get rewarded with an S for it but i didnt..... it crushed me.... ;_;