I beleive that most of the balancers are designed so that the tire is placed horozontally (lugs are verticle) with eh face of teh wheel facing up. Placing teh weights on the bakc of the rim would require the operator to remove the wheel and crimp them in teh correct spt, then place the wheel back in the balancing machine.
Long story short. It would take the operator (mechainic) longer to balance the wheel that way.
It's also easier to see if a weight is still in place or has come off. But I've never had a weight come off a a car that I'm aware of.
rhird reason. The wheel offset often places the face of the wheel closer to the mounting point. Therfore, if you place th eweight on the bakc of the rim, you might cause stress of the bearings, as a slight wobble could develop in the lateral axis since hte weight is so far laterally from the mounting point (center) of the wheel.