This was primarily done to reduce weight- Simplifying the mechanism resulted in significant weight savings. Also, I'm not sure why the wings were called 'heavy'. While the SP used the same wings as the legacy 747s, the internal structure and construction was modified significantly for weight savings. As this site notes:
While Boeing chose to retain the standard 747 wing on the SP the internal structure was significantly modified to reduce weight. To save weight Boeing employed reduced gauge metals in the wing spars, ribs, skin and stringers in the wing box and center fuselage sections.
In an effort to further reduce the weight of the wings engineers designed a single-slotted variable pivot flap system. ... (which) ... removed 12,000 lbs in weight from the wing structure.
Considering the total weight savings was around 45,000 lbs, this is more than a quarter of it!
