Two things:
1) Mutually Assured Destruction. Team Obama was in possession of a bulging dossier on the Clintons.
2) Obama had the black vote already sewn up. If Hillary had won the nomination, they would have stayed home out of protest, giving the election win to McCain. Neither the DNC or the RNC wanted that outcome.
Didn't Axelrod work for Clinton before switching? He'd be the likely conduit then.