Bill Scher observes that Joe Biden, Bernie Sanders, and Elizabeth Warren all made implicit promises to their backers that were rationales for their campaigns. Biden said he had an unshakable bond with black voters. Sanders said he would generate big increases in turnout among young voters. And Warren said she had a detailed and convincing plan for everything. But youth turnout hasn’t jumped and Warren wasn’t able to come up with a health-care plan that made even superficial sense. Only Biden delivered, and that’s why he’s winning.