Why do some A380 main wheels have no brakes?
Because they are not needed (for the passenger variant). Addition of brakes increases the weight and complexity of the system, which is undesirable. Why add something that is not required?. If it is required, additional brakes can always be added, like in the (proposed) freighter version. According to flightglobal.com:


  Sixteen of the A380-800's 20 main wheels are braked, with the two aft wheels on each body landing gear of the passenger version not having brakes (unlike the freighter variant, where all 20 main wheels are braked due to its increased weights). 


Also, Aft Axle of BLG is steerable to ensure better maneuverability for small radius turn (Body Wheel Steering); braking a steerable axle would require special linkages, increasing weight and complexity. This is not insurmountable, however- Boeing 777's landing gear is similar, but has brakes on all its wheels.