There exists a group of people who have always wanted what he's offering. And this is the first time they've had a candidate who is that far left.
The Dem party knows that someone like that cannot win over the moderates in a general election; so they've always tried to block them. I don't think they can this time. When Bernie wins the Dem nomination, I predict that the money-people behind the MSM will decide they like him even less that Trump and that they'll order their talking heads to switch sides. That is why I suspect they are so worried about Bernie: not just because they know it means for more years of Trump, but because they'll be forced to help it happen.