Since the normal force is at an angle, it now has a vertical and horizontal component. This horizontal component force now contributes to the centripetal force. The frictional force is parallel to the slope/banked track so it also possesses a horizontal & vertical component, and the horizontal component also contributes to the centripetal force. However, this horizontal component is smaller than if the car was on a flat road. On a flat road only friction contributes to the centripetal force. Hence, on a banked track less frictional force is required, for the same centripetal force. To answer your question, the frictional force is reduced because the normal force now contributes to the centripetal force.