Not familiar with A380 flight controls, but one reason will probably be as with the 747. Outer wing ailerons are too powerful for high Mach No. cruise and would require outer wing stiffening. The 747 reverts, when flaps are retracted, to only inner ailerons, which conveniently can be placed in that space at the back of the wing behind the inner engine, where flaps cannot be placed. These inner ailerons alone are enough for roll control at high speed. The outer aileron in the A380 may be split into two for structural reasons and hydraulic redundancy.