What determines belt side is the direction the motor rotates. Since you always want the brush to sweep towards the back of the nozzle, if the motor you chose to implement into the design rotates one way or the other, that determines where the belt goes. Early Panasonics had the belt on the right side of the nozzle as well, as do Dyson DC07, 14, and 33 machines. Left side belts are the most common, probably because counter-clockwise rotating belt shaft motors are what is most common, I think it also lends to right handed users being able to get good edge cleaning along the right edge.