It's really not that shocking for people who are actually on the left, to want political representation that is actually on the left. The Democrats are a right of center party, that has a monopoly on the left because there is nothing to the left of them. The only areas where Dems like Biden and Clinton are left wing, are on some social issues. That's the only area where they are even remotely progressive.
Being more progressive on some issues than a very religiously driven ultra conservative political party, doesn't make you left wing, or progressive in general.
The political spectrum in the US is a rigged game currently. It's like picking jurors from a list of jurors the mob has already bought. Your choices more often than not are religious conservatism, or conservatism.