Why does Beowulf have to battle Grendal with his bare hands?

Like many heroic battles the protagonist (Beowulf) must be compared to the antagonist (Grendel) in the most even physical level to demonstrate that he wins because his cause is just not because his technology is better. The same imagery is used in most westerns (the Morality plays of the American cinema) where it is the hero that brings "a knife to gun fight" to demonstrate that his innate goodness is his real strength.

In this case Beowulf is the new hero of Christianity coming to the wilds of Scandinavia, Grendel is the evil holdover of the heathen past.