Because that Bloomberg endorsement is sure to push Obama over the edge.

Via Politico:

Biden’s gift for reading people played a role in the endorsement. He and Bloomberg have been friendly for years, and have played golf together. The V.P. had called the mayor over the weekend, and the had talked several times this week about Sandy. Then yesterday morning, Biden was in Davenport, Iowa, about to go to church for All Saints’ Day. Finally, he made the ask, nudging Bloomberg toward the endorsement: “Mike, there’s too much at stake. We really need you to do this.” And the mayor did.