[Lil' Lavery]

So many people have asked why, and I'll try and elaborate on what Billfred and the members of 703 have said.
This design effectively emulates many positive features of a treaded drive system, as well as eliminates some of the negatives of chain and belt driven drives. The close proximity of the wheels allows for the robot to have a very, very, low ground clearance and still have the ability to climb ramps, steps, and platforms without bottoming out or high centering. The greater quantity of wheels creates more contact area with the ground, which in turn allows for a more even distribution of weight (although, still not nearly as well as a tread in this respect). The fact that all 14 wheels are driven allows for any wheels to lose contact with the ground (such as when traveling up an incline) and for the robot to still have the ability to drive. The use of intermediate idler gears instead of chain or belting eliminates the risk of the chain/belting popping off or slipping. It also may have actually saved weight depending on the size of the sprocket/pulleys and the chain run (if they would have chosen a "staggered" chain run, so that a single chain failure doesn't result in the failure of the whole side of the drive, it probably would have weighed much more than the gears).
There are some cons to this drive system as well. Spur gears are slightly less efficient than chain (not by much though), but so many gears magnifies that slightly. Additionally, while failures should be far less often, if/when they do occur, it is likely to be far more spectacular and require probably a more complex fix than if they had used chain or belting (although, once again, this design was chosen to try and make sure failures wouldn't occur). Additionally, the 14 wheels themselves are far heavier than the 4 or 6 pulleys they would have likely used in a tread drive.