It wasn't arbitrary. The states that had pre-existing laws kept them. The original decision prevented states from passing new laws to enable gambling, but did

It wasn't arbitrary. The states that had pre-existing laws kept them. The original decision prevented states from passing new laws to enable gambling, but didn't strike down or repeal existing laws, thus those states essentially got grandfathered in. Media reports like to spin it as "sports gambling is illegal except in XYZ states", to make it look like favoritism and the story inciteful, but really the only thing distinguishing those states is legacy code.