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.
The Danes are again overjoyed, and Beowulfâs fame spreads across the kingdom. Beowulf departs after a sorrowful goodbye to Hrothgar, who has treated him like a son. He returns to Geatland, where he and his men are reunited with their king and queen, Hygelac and Hygd, to whom Beowulf recounts his adventures in Denmark.