1) People vote for a President, not a Vice-President,
2) A significant portion of the pro-Obama primary vote was a “I-am-sick-of-the-Clintons” vote! Many democrats are tired of the Clintons.
3) An Obama-Hillary ticket would energize the GOP base like nothing else!
4) It would look weird !
The Beastpeople will automatically go with the ticket. They will not migrate in some reasonable percentage to McCain. McCain already has a good portion of the ‘I am sick of the clintons’ vote.