In layman's words
Because when you view from the back of an aircraft, the resultant of the flow on the upper and lower wing causes a swirling type of motion that twists inwards (clockwise for PORT wing and Anti-clockwise for STARBOARD wing) called vortices.
So I suppose by having winglets bent upwards, it assists in "smoothing" out the upward flow of the vortex as it leaves the wing tip and due to viscosity, the vortex will "stick" to the winglets. Drag is thus reduced.
Whereas having a downward bent winglet has the effect of "stopping" the continuity of the vortex flow but unfortunately results in additional drag.
Can you picture it ?