I think as far as these 4-door cars are concerned, the Hemi wasn't a suggested engine option so you had to think outside the box to get one. Only 4 people found it necessary to bother. The couple of articles I've read on these it was suggested that they were ordered for towing capacity by older guys. I can't imagine that a Hemi wouldn't be a good engine for towing back in the day but the wedge motors were no slouches and were part of towing packages already.
As far as '71 Hemi verts? I think the mounting insurance costs and rising fuel prices were enough to extinguish those numbers. I mean how many were there in '72???