Because he is a good leader and responsible and he protects his country and willing to obey his rules
King Arthur is a good and kind-hearted leader. He is selfless. Even though that his wife and his son have done bad deeds to him, but still he loves them. He is also brave and courageous, for he is not afraid to show himself into battle with his rivals. In other words, he is fit to be called "King". 
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