Each tire can only hold a limited amount of weight. If you take out two tires in the middle, the remaining tires could not support the weight of the truck. Also Articulated Dump Trucks run on soft soil and slippery roads by driving all six tires. It would not run well on those surfaces without the added traction of the middle tires.

Dump truck which can move tightly [THE GREAT PICTURE BOOK OF CONSTRUCTION EQUIPMENT]