With the exception of the late 12" cabinet models that came out circa 1959, all Atlas lathes were intended to have the motor mounted behind the countershaft and behind the bed. So there would have been no incentive for the added cost of providing space underneath the headstock to pass the belt through. In the case of the late cabinet models, the spindle pulley is about half the diameter of the largest step on the 4-step spindle pulleys on all previous machines except for the original 9". So the clearance through the bed underneath the spindle is significantly narrower on the 12" under drive models than it has to be on any of the others. The two 4-step pulleys are mounted elsewhere.
I will add in passing that, although it would take a rather peculiar set of circumstances to make it financially attractive, there is no reason why one could not put the double belt pulley off of a late 12" cabinet model onto the spindle of an Atlas 10" or 3/8" bed Craftsman 12" and make them into cabinet models.
