Bernie couldn't win when it was just Democrats deciding amongst themselves.
The Democrats ran Bernie before, during a time that was much more favorable for a progressive candidate. Only then, his name was George McGovern and he got beat like a rented mule.
Progressives need to learn how few people in this country are willing to consider the notion of the possibility of thinking about letting their daughters date a progressive, much less elect one President. It's not enough to be right, if you're fucking stupid about the citizens of this country.