It also depends what the martial art wants to see happen in fights. So if you want to see high kicking, because that is what you do, you allow high kicks in competition. This happens a bit with things like limiting ground time to promote throws but discourage ground work. There are even competitions that say you have to throw X amount of kicks per round. Otherwise I can see why head strikes bare knuckle are discouraged. People would come out of fights looking carved up. Most people I know who have done bare knuckle suggest it really isn't safer because people don't hit as hard.