As much as she's lost, she's also tantalizingly close.
Anybody who wonders why Hillary Clinton hasn't quit needs to take a couple seconds in her shoes. She's 160 delegates down with 400 delegates left. But, she knows as well as anybody else that the Democratic Nominee is going to be president. Either of them would probably win with ease. That said, would you ask that McCain stop his campaign in October? All your really mean ads are going to hurt the unity of the country and you don't stand a chance as you're polling down 15 points! Do you think McCain could or would stop? Now look at the primary again. Either one of them would probably easily trounce McCain, who eeked out a win against a pathetically weak bevy of opponents ranging from Sleepy McEatTerrorists to Rev. LetsRewriteTheConstitutionIntoTheBible and nobody should forget PayeeMcCash MagicUnderpants.
I doubt even the worst she could toss at Obama would damage him enough to lose. As much as people love to say they wouldn't vote for the other person in the primary, the alternative is to vote for a third term for Bush in a much more decrepit packaging.
If she drops out she'll have to wait until 2016 to run again and even then Barack's VP would be presumptive and she'd be downright old.
One shot, and you need 300 of 400 delegates... to become president or you very well might never have another chance. Would you quit? What if you were probably going to lose?
-- That said, she should totally drop out. She's become the creepy ex-girlfriend of the Democratic party. Sorry Hillary, stop calling... we're with Barack now.