Final answer:
Beowulf's armor is impenetrable, which is why the knife does not kill him. This situation represents symbolism.
Step-by-step explanation:
The ultimate reason that the knife does not kill Beowulf, according to the narrator, is that Beowulf's armor is impenetrable. This situation portrays the literary element of symbolism, as the armor represents Beowulf's invincibility and protection against harm.