One of the key items for Obama’s lack of support for a Michigan re-vote was that the voters who crossed over party lines to vote in the republican primary wouldn’t be allowed to participate. Since he had voluntarily removed his name from the Michigan ballot, it’s likely several of his supporters crossed over to vote in the republican primary on the same day. They wouldn’t be allowed in a re-vote apparently because of some DNC party rules.
The fact is that even though he had said he would “fine” with a new primary in Michigan if it could be done in a way that gave him and Senator Clinton time to make their respective cases and the DNC signed off, he eventually realized his campaign would be at a disadvantage because of his decision to remove his name from the ballot.
Supporting a revote just be giving Clinton an opportunity (and advantage) she wouldn’t otherwise have, and he wasn’t required to do so. He is in a much better position today than he was January 15th and would likely do much better with June primary, but why take the chance?