A constitution is a statement of the basic principles and laws of a nation, state, or group, such as the U.S. Constitution. Another very common meaning of constitution is the physical makeup of a person. So think of a constitution as how a body (yours, the government's) is set up. If you have a strong constitution, it means you don't get sick very often.
I think the focus of the story is on Beowulf as a hero rather than a king because it is his heroic deeds that lead to him being offered the throne. Also to be king one must possess some type of heroic qualities in order to rule a land and its people.
Beowulf is a classical hero, who encompasses strength, bravery, intelligence as well as other characteristics. Sir Gawain also displays these same qualities, but he is a direct example of the chivalric hero. Knights of the chivalric code are noble, loyal, courteous, and chaste, all of which describe Sir Gawain. Beowulfâs bravery is admirable, even more so that he gives most of his credit to God.