Maybe it's one of those situations where so much of the pork is consumed by humans, it would be more difficult to make enough available, cost effectively, to the dog food industry. Think about pickled pigs feet, etc. And we do see a lot of pork necks and such in the grocery stores. Think about all the different things that are made from pork such as sausages, bacon, etc. where the not so prime cuts are used.
Just speculation, but, who knows.