Got 3/1/15 and got a B-, why are assist not rewarded?

Hello, so I was playing as Rammus and went 3/1/15. When the game ended I got a B- after getting several S' in a row because I got more direct kills. Riot, this is a team game, assist are almost more important than the actual kills. Without assist, half the kills in the game wouldn't of happened. So why are we not rewarded with a better game or more IP for being cooperative with kills and our team? This seems ridiculous for a TEAM game. Because kills are much more rewarded, people on the same team fight over getting and assist or a kill. This isn't right for a team game.

EDIT: I went 5/1/10 and got an S- and more IP, doesn't seem like much of a difference does it?