As has been pointed out, the rate of fire, and ease of loading were considerations for the military. The military smoothbores were also designed for the use of the bayonet, which the rifle was not. From a civilian stand point, the smoothbore was much more versatile for frontier defense and putting meat on the table. The smoothbore could serve as both shotgun and short range ball gun for the person who for what ever reason, could only possess one gun to do it all. Inside of 70 yards, you can hit game or men with a patched round ball just as well as you can with a rifle. The military of that time was predicated on putting the most lead into a given area in as short a period time as possible. The smooth bore will do that much better than a rifle will. In the case of the Americans, buck and ball is only possible from a smooth bore.