How come no A bodies on Graveyard Cars

Plus, the average viewer doesn't relate to the Dart or Barracuda nearly as much as the Challenger, Charger or 'Cuda.

They couldn't care less about "relating" with viewers. The show is just marketing for Mark and his place. Also, if you haven't caught on through watching the show, he needs feel like the smartest person in the world. Being able to tell everyone facts even if they are not and getting them to believe you will make you the smartest person.