The flow around an airfoil has a stagnation point where the flow seperates to go over and under the airfoil. As the angle of attack increases the stagnation point moves down and aft from the leading edge a little. With a sharp leading edge, the flow over the top of the wing must first negotiate the sharp leading edge. The sharp leading edge causes turbulence and with increasing angle of attack, seperation of the flow resulting in premature stall. As angles of attack increase, a rounded leading edge allows the flow over the top of the wing to stay attached longer delaying the stall.