No S grade because of surrender???

The Painbringer would like to know how having a 31-1 kda in a ranked game does not merit an S grade.
If it is because of enemy team surrendering that is incredibly poor grade system - they surrendered as we were nuking the nexus. I would really like some official explanation on the matter. I did everything right. My teammates were very cooperative and we had a strong hold on victory from lane phase right through to the end push. Everyone did a great job and worked together and I'm positive my role was very impactful on the success of the game.