Why did J.R.R. Tolkien describe Beowulf as more like an elegy than an epic?

J.R.R. Tolkien suggested that the total effect of Beowulf is more akin to a long, lyrical elegy than a traditional epic because of its pervasive sense of sorrow and inevitable loss.1

While the poem contains heroic elements, Tolkien noted that it is filled with ominous references that foreshadow the destruction of the characters and their world. Even in the poem's earlier, more celebratory sections, there is an underlying awareness that such joy is fleeting.1

This elegiac quality is further deepened in the second part of the poem, where the narrative movement becomes slow and funereal. As the aging Beowulf faces his final battle, the poem contrasts his past triumphs with his impending death, creating a somber mood as the wyrd (fate) that claims all men closes in on him. By setting these marvelous tales against a historical background where victory is always temporary and strife is constantly renewed, the poet gives the entire work an elegiac cast.12