Hillary is the least trustworthy candidate running. She has ongoing scandals which could erupt again at any moment, a philandering husband that has a laundry list of dodgy business dealings, a foundation that is mired in controversy, an ability to rev up the right-wing base that's unmatched, and numerous other weaknesses related to her banking connections, her Goldman handcuffs, her weapons deals at State, her disastrous decisions at State, her vote for the Iraq War etc., etc., etc. etc.
On the OTHER hand, the Dems are coming out in HUGE numbers and donating in even larger numbers to Sanders. He's the only candidate with a net favourable rating, and Dems - and even some Republicans - say that they PREFER "socialism" to capitalism, because Sanders opponents call him a socialist, so they think he is, and yet they love what he's saying. They don't care about that label, because it's not the 90s, or the 50s anymore.
Plus of course, a large number of Sanders opponents are now fully aware of Hillary and her corruption. They aren't interested in her, at least a percentage won't be. And of course, she can't get the youth to believe in her either. And they vote now, if you haven't noticed.
Clinton is basically acquiescing to whomever the right nominates. And betting that all the polls are going to magically change after the nomination is wishful thinking and nonsense.