@snood,
Bernie Sanders didn't construct the grassroots movement that grew up around him, and he doesn't control the people who comprise it. He only has the power of the progressive movement as long as he earns it, as he has been doing.
If he were to call on his followers to support Hillary, he'd lose the trust he's earned and he knows it.
It would be up to a candidate to change voters' minds. Since dishonesty is her primary trait, Bernie supporters of the Bernie or Bust variety would likely not believe anything she would say.
Bernie would be considered dishonest by his followers if he threw in behind Hillary.