You were playing Quinn. You should not have had any trouble with doing both. Especially, if it's supposed to be a S+, an absolutely stellar game.
"Why should I not get S+ for not being selfish?"
Because an S+ isn't a grade of just not being "selfish", which is what you are supposed to do. And both of those are what you are supposed to do, but you did not perform well in one of them.
Serious question:
Why do you think you were supposed to get not just an S tier grade but an S+?
Are you hung on that you went 11/1? There's more than K/D ratio which isn't even that important in this game. There's plenty of stats that you can be graded on. Like kill participation. You say you were too busy roaming to CS, because you needed to help, your kill participation should be great then. It's about 4 out of 10.
Damage dealt to champions, should be a lot; you were fed, you were 11/1. Your damage is only 1.7k more than the Karma who was 2/9.
"Wards placed", vision control is important. You placed 5.
Gold obtained. To compare it to another Quinn game in your bracket, you didn't earn even 500 more gold than a Quinn who went 3/11.
What about this game makes you think you played better than everyone else playing Quinn?