Probably because you had booty cs.
Edit: Yes, you had booty cs. 5 cs/min.
Edit#2: You only dealt the 3rd highest damage in game.
Edit#3: You placed the 3rd FEWEST wards in the game, and least amount on your team.
Edit#4: You took 1 tower, and even gave up first tower blood to your lane enemy.
Basically, you didn't get an S because you are confusing Call of Duty with League of Legends, and therefore think the game is 100% based on the number of kills you got. Yes, you played really well that game. Not debating that. But unless you have a complete game, you're typically not getting an S.
Edit#5: As for comparing to Vel, he had 100+ more cs than you, 18,000+ more damage than you, placed 6 more wards, killed 1 more ward, placed 1 more control ward, had 1k+ more gold than you, took 1 more tower, and took 1 more inhib than you. If you don't think all of that equals 4 kills, I don't know what to tell ya lol. His KDA was still over 5+, which is more than enough to get an S rating.