In addition to what Mori already said, here's why:
1. Either one of these superb politicians would do more good in the Senate than they would in the largely symbolic vice presidency.
2. The question of who'd be (ahem) on top just doesn't have an easy answer. Obama's charisma would be wasted in the number two slot, where he'd be loathe to upstage the presidential candidate -- but so would Clinton's policy chops. Having a better health care plan wouldn't do her very much good if she had no real authority to implement it. And it's naive to believe that as president, either of these two would allow the other too close to the reins of power.
3. I mean, really, could they work together after all this? Get a Clinton in the V.P. slot and the entire Clinton camp would set itself up as an alternative politics and policy shop to the West Wing. That wouldn't be Party unity, it'd be Party competition.
--Dana Goldstein