To elaborate a bit on what SMOC wrote:
The wing on the 747-8 is aerodynamically pretty much all new relative to the -400 ('structurally similar but aerodynamically new'). As a result of the new aerodynamics, the pressure distributions across the wing changed rather dramatically, which in turn lead to large increases in many of the control surface loads.
The decision was made early on in the development that, as long as the spoiler and outboard aileron systems needed to be redesigned, we might as well go with FBW systems - which also made it easier to mimic the characteristics of the -400.