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, Sir Gawain and King Arthur - Three Heroes. The three heroes discussed here, Beowulf, Sir Gawain and King Arthur, are heroes for different reasons. Beowulf, our earliest hero, is brave but his motivation is different than then other two. To Sir Gawain personal honor and valor is what is important.