This is pure speculation...but maybe Al's Q rating and internal polling was so poor after his latest round of appearences that he had no choice but to accept reality. Al Gore is simply not likable...his wonky policy statements, wardrobe confusion, faux hipness and forced cheer is not sitting well in middle America.
The irony is that Al's best and worst bit of luck was to become enmeshed with the Clintons. They brought out his worst impluses, while propelling him center stage where the electorate could witness his lack of grace and judgement.