Round ones take up more space per square foot, so I don't know why they'd use them at all commercially unless someone thought they'd work better. Like you, the only round ones I've seen are for Dungeness crab. Maybe they have the square pot phobia?

The Bering Sea guys seem to have done a pretty good job of wiping out that fishery, so the big rectangular pots must work just peachy for King crab, Tanner crab, etc.