Why didn’t Dr. Mortimer recognize Sir Charles at first?

From the text:

On its jagged face was spread-eagled some dark, irregular object. As we ran towards it the vague outline hardened into a definite shape. It was a prostrate man face downward upon the ground, the head doubled under him at a horrible angle, the shoulders rounded and the body hunched together as if in the act of throwing a somersault. So grotesque was the attitude that I could not for the instant realize that that moan had been the passing of his soul. Not a whisper, not a rustle, rose now from the dark figure over which we stooped. Holmes laid his hand upon him and held it up again with an exclamation of horror. The gleam of the match which he struck shone upon his clotted fingers and upon the ghastly pool which widened slowly from the crushed skull of the victim. And it shone upon something else which turned our hearts sick and faint within us—the body of Sir Henry Baskerville!

There was no chance of either of us forgetting that peculiar ruddy tweed suit—the very one which he had worn on the first morning that we had seen him in Baker Street. We caught the one clear glimpse of it, and then the match flickered and went out, even as the hope had gone out of our souls. Holmes groaned, and his face glimmered white through the darkness.