Think of the aerodynamic forces. The stagnation point on the wing will always be somewhere on the LEDs and you are attempting to move them in the face of the full airflow at up to 250kts IAS in the case of the first stage of flap on many aircraft.
Think also in the case of Kruger flaps the amount of extra surface area you are attempting to present to the airflow. I know nothing of the dynamic change in aerodynamic forces as the wing shape changes but I would imagine it is quite impressive. I would also guess that in the case of slats, both the relative airflow and the flow acceleration in the gap would tend to try to close the gap.