Liberals feel obligated to vote for Hillary, for a variety of reasons. Chief among them are her perceived electability advantage over Sanders, the fact that she's a woman, and her length of service in politics. But in their heart of hearts most of them would prefer to vote for Sanders. The longer Sanders sticks around and remains viable, the more they'll listen to their hearts. Everyone seems to think Sanders can't win, but no one thought Obama could, either.
Of course, Sanders isn't a hip, young, fresh-faced newcomer either.