Mark Halperin makes a list:

1. She can’t win the nomination without overturning the will of the elected delegates, which will alienate many Democrats.

2. She can’t win the nomination without a bloody convention battle — after which, even if she won, history and many Democrats would cast her as a villain.

(…)

6. Many of her supporters — and even some of her staffers — would be relieved (and even delighted) if she quit the race; none of his supporters or staff feel that way. Some think she just might throw in the towel in June if it appears efforts to fight on would hurt Obama’s general election chances.

(…)

9. Many of the remaining prominent superdelegates want to be for Obama and she (and Harold Ickes) are just barely keeping them from making public commitments to him.

10. She can’t publicly say more than 2% of all the things she would like to say about race, electability, beating McCain and experience.

11. If she somehow found a way to win the nomination, she would have to offer Obama the veep slot, and she doesn’t want to do that.

12. This is a change election, and Bush-Clinton-Bush-Clinton can never truly be change.