Beowulf is written in Old English. It is also an epic poem.
Beowulf's presentation as an epic poem emphasizes certain qualities within the narrative. Firstly, like most epic poems, it valorizes a mythic past and details the adventures of a mighty hero. The poem presents a pagan Anglo-Saxon past (though not without some Christian allusions sprinkled throughout the narration). Beowulf himself is a sterling example of what the Anglo-Saxons valued in their heroic characters: he is brave, strong in leadership, generous toward his faithful warriors, and wise in old age. Epic heroes tend to not be ordinary people or nuanced figures: they are larger than life and meant to be put on a pedestal.
Secondly, the epic presentation was designed to make the oral presentation of a work easier to memorize and perform. These works were usually read aloud. Therefore, the writing style has a rhythmic and dramatic edge to it that a novel might not. Notice also that the work is written in verse as opposed to prose for this reason.
Overall, Beowulf is told as an epic poem for two reasons: epic poetry was a prime form of storytelling during the time of the story's composition and the narrative features of the epic poem are well suited to the archetypal and larger-than-life qualities of Beowulf as a narrative.