I'm sure it has to do with overall balancing of the wheel/tire assembly. Probably to minimze the weight required to balance the assembly.

Back in the "good old days", tires were marked with a yellow dot to indicate the "high spot" of the tire rotational balance. That was suppose to be aligned with the valve stem where the "low spot" was for the wheel. "Best Practice" has all but gone by the wayside nowadays. Partly with the advent of TPMS (which changed the rules); partly because a tire changer at most multi-service shops is the entry level position. You just about have to go to a Discount Tire shop or something similar where all they do is mount tires & wheels. Those guys usually know the optimum geometry for mounting a tire to a wheel. You really want only the minimum weight necessary to balance the tire/wheel assembly.