Why do you think the author chose to rewrite the story of beowulf from grendel’s point of view?

It is called the third person or the Omniscient.  To present Beowulf's thoughts, for instance, the narrator adopts the point of view of an omniscient author and gives us a glimpse into Beowulf's mind; or he causes Beowulf to reveal his private thoughts by having him declare them through a public utterance. In either case, a reader's attention is directed toward Beowulf.