Nobody, except Bernie Sanders supporters, thinks that the senile Socialist is pulling ahead on charisma, messaging or ideas. Aside from ranting about corporations being all corporationey, Bernie Sanders doesn't have any ideas, policies or any plan to pay for it all. He's a 19th century leftist clumsily tagging along after the last wave left on a variety of politically correct issues like #BlackLivesMatter.
Hillary Clinton could explain losing to Obama or even Elizabeth Warren, and certainly Joe Biden, but losing to Bernie Sanders isn't a referendum on Bernie. It's a referendum on Hillary. And she's losing because people just don't like her.
The hard left is just ideally positioned to take advantage of that dislike. If they can take advantage of it with Bernie Sanders, then they could take advantage of it with a confused goat. It's all one and the same.
Bernie Sanders doesn't have some magic formula. The closest thing he had to it was authenticity and that went out the window as his campaign took off. The current Bernie Sanders model is as authentic as a Toyota mocked up to look like a '57 Chevy. Currently Bernie Sanders will adopt any set of talking points his people hand him and mumble through them, assuming they get to him in time. If they don't, he says the sort of thing that might have made sense back when the left was slightly saner, e.g. police should investigate rape, only to be dogpiled on because it's not the current left-wing PC orthodoxy.
The magic here isn't Bernie Sanders. It's Hillary's anti-magic, her anti-charisma and her anti-likability.
Hillary Clinton would be having just as much trouble with any politician to the left of her. She would be having just as much trouble with any alternative. Because when people hate you, any other option is one option too many.