Hillary scores high in polls, but, other polls suggest that even many Democrats have Clinton fatigue. Sad to say, many liberals perceive Bill and Hillary as being too conservative or too big business oriented for their liberal tastes.
Clinton has baggage even Democrats can't stomach. She was eclipsed by a flash in the pan in 2008, and looks like it could happen again. But Elizabeth Warren? I guess you get a twofer. First woman president, first Native American president.