Two reasons:
1) From Obama’s point of view: There is no guarantee that it will unite the Democratic party, since those who were willing to support Clinton in the top slot might not do so with her in the second chair. There is a likelihood, however, that her presence could galvanize the GOP against her, which is exactly what Obama doesn’t want since McCain remains a candidate with soft support in the GOP. And it is problematic for Obama to sell himself as an agent of change when he’s got the symbol of Washington insider status quo as his running mate.
2) From Clinton’s point of view: According to the “West Wing” scenario, the young, vibrant, ethnic candidate chooses a Washington insider with close ties to a previous administration (and has one daughter) as his running mate who then dies on the eve of election. Why risk it?
PAD
• 
Not so shocking looking at his weight. Severely overweight people tend to die much younger. As do tall people. Short, skinny women seem to live longest.
My dad is almost ten years older than Russert, Alan, and was carrying well over a hundred extra pounds for about twenty of those years. I’ll stick with “shocking”.
TWL
Bill Mulligan: “But Jason…Obama DID throw away his relationship with Wright.”
Yes, but not after one or two comments. Obama showed great forebearance toward his former pastor, which was repaid with multiple slaps in the face in the form of Wright’s deranged behavior during a news conference. Only then did Obama cut him loose, and it was clearly painful for him to do so. Jason is therefore correct.
Alan Coil: “Not so shocking looking at his weight. Severely overweight people tend to die much younger. As do tall people. Short, skinny women seem to live longest.”
Tim Russert was a highly respected journalist, and deservedly so. He also had a reputation for being a genuinely decent human being. I choose to focus on that, and not his physical appearance.
Rest in peace, Mr. Russert.