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.
3. Catching up in the popular vote is not out of the question -- but without re-votes in Florida and Michigan it will be almost as impossible as catching up in elected delegates.