Slope of SML

Why is the slope of the SML the market risk premium and not Beta?

Beta tells you how “exposed” you are to the market risk premium. The acual slope tells you how much return you get per unit of systematic risk.

Beta is on the X- axis. The E(MRP) is the slope because it shows how much the expected return will change per unit change in Beta. E® = Rf + E(MRP)*Beta