Because Hell, for Dean, wasn’t just being downstairs in lightless agony, forever. No. Hell for Dean Winchester was the thought of burying his brother’s corpse.
And for Sam, Hell wasn’t just an eternity in the Pit. Oh, no. Hell for Sam Winchester was his brother dying at the end of his fists, his brother fighting to the bone and losing against the cosmic forces of good and evil.
Hell isn’t a place for the Winchesters. It’s a state of being where the other is gone.