You're correct instating that Beowulf is focused on as a hero rather than a king. You've asked two questions here:
- What is the difference?
The question of "why" is best answered by looking at the purpose of the poem itself. It is written as a heroic epic, a tale that focuses on the heroism of an individual (or group of individuals.) Therefore, the story must be about the heroic deeds of Beowulf rather than his kingly duties. What fun would a poem be about Beowulf passing laws, or attending diplomatic banqets? Aside from that, he doesn't actually become king until later in the book.
This sort of answers the "what's the difference" question as well. A king doesn't have to be brave (that's why kings have knights.) That's not to say that there are no warrior kings (just look the Conan the King comics) but the two don't have to go together.