The pitch distribution is shaped in this way in part because the pitch is measured from the bottom of the airfoil. If The pitch were measured relative to the "Zero Lift Line" the graph would be fairly straight from the root out to about the 3/4 chord point. The curvature on the graph below is just what it takes to keep the airfoil at the angle of attack for best lift/drag ratio (L/D).
At the tip the effect of Mach number is to reduce the angle of attack for best L/D. This effect and the tip losses reduce the pitch near the tip.
Two cases are shown. Case A for a high performance F2C and case C for something tha is about 85% as fast. The lap times are based on an effective line length -- (line length plus pilot contribution) of 16.32 meters.