People are scared off by the "socialist" label. Even though he has explained numerous times what he is as a democratic socialist, people are still too ignorant to really think about what that means. Look at this site as an example of that. "I'm not voting for that crazy socialist!" But they can't say what they have against any of his specific policies. This is what is keeping him back from beating Hillary.
I am not a democrat but would gladly vote for Bernie and genuinely be excited to have him in office. However when Hillary wins the nomination I'll vote for her simply to ensure we don't end up with regressive supreme court justices. Beyond that, I don't trust her for shit and she'll be more of the same we've had for far too long in this country.