Why does lift curve slope increase when Fowler Flaps are deployed?
Note that your plots show lift coefficient over angle of attack. This lift coefficient is referenced to a reference area which must be defined somehow. It is customary to use the projection of the clean wing area in the x-y plane for all flap settings in order to keep coefficients comparable. The lift curve slope increases because the real wing area increases while the reference area remains unchanged. The dashed line in the lower plot shows the isolated effect of camber increase while the upper solid line combines both effects (area and camber increase).