Here's why:
* Physical Presence: The raven is a constant, looming presence throughout the poem, both visually and aurally. Its black plumage, "grim, ungainly, ghastly, gaunt, and ominous," contrasts sharply with the speaker's surroundings.
* Symbolism: The raven goes beyond a mere bird; it embodies the speaker's grief, despair, and obsession with his lost love, Lenore. It represents the darkness of the speaker's mind and the relentless nature of his sorrow.
* Repetition: The raven's single, haunting word "Nevermore" underscores its symbolic power, repeating and echoing throughout the poem, driving home the speaker's despair.
* Focus of the Narrative: The poem's narrative revolves around the speaker's interactions with the raven, his questions to the bird, and his increasingly disturbed reactions to its replies.
While other images like the speaker's chamber, the bust of Pallas, and the dying embers of the fire contribute to the poem's atmosphere, the raven stands out as the central and most powerful image, shaping the poem's themes and driving its narrative.