Because he wanted out of politics, had his new eco gig, and plainly lost the itch for politics. He would have to compete with Hillary and Obama. Gore would split the regular and conservative Dems with Hillary, and Obama would sneak up the middle while drawing from both. That is if Obama runs. Obama is a regular, but more attractive (until recently, ex DADT) to the Dem left than Hillary. Gore is a SoDem, almost Blue Dog. Don't think the militantes are crazy about them.