Because the ranking system is a big fat failure and I believe Riot have long since gave up on making it accurate.
I've had hard carry game with A+
Then games which I rode my team mates dick to victory with S- all the way to S+
Quite frankly it doesn't, and can't measure your impact on the game. It doesn't even compare relatively how good/bad your team/opponent is.
The system only give S ranks for facerolls. When it does so, it does it en mass, Oprah style. Then it used said faceroll stat to say that your hard carry Xayah game wasn't as impressive as another when a blue shen ran down the turret feeding said Xayah 20/0, because 12/4 at 30 minutes <<<<< 20/0 at 20 minutes
Don't care too much about it. If you specifically want S rank for chest/token, do normal and hope for Riot match making to fuck up. Which, in my experience, happens 3/4 games.