They're easy to understand: they are privileged and insulated enough from potential Republican fuckery that if Democrats lose elections, they are mostly going to be ok. They're mostly middle class or higher with secure employment or other economic support (parents, spouse), straight (or closeted), their religious status isn't contentious, mostly they're male.
The other group I'd expect would be extremely low information voters who believe all politicians are the same and it's a Coke or Pepsi kind of thing. That's not going to be super common with anyone identifying as "progressive" due to the very politicized nature of that term, though.
Yeah, except Bernie's supporters voted for Hillary at higher rates than Hillary folks voted for Obama, so maybe Hillary, her dumb fucking insecure email server, her lack of personality, and the DNC who helped her cheat her way through primary debates are the real ones to blame, not the progressives. She was a terrible candidate who the right hated and independents didn't like much better and she proved she was a terrible candidate by losing to the worst Republican candidate in the last hundred years. She had all the help she should've needed from progressives.
Looking into it more, it appears you are likely right. Was probably the echo chamber I was in while I was younger telling me otherwise. Seeing a lot of "Bernie Bros" and the like saying they wouldn't.
I just have a perspective beyond myself, rather than your liberal myopia.
Socialism or barbarism. Either the American empire is stopped or civilization will come to an end, either by climate change or world war. The point of Cornell West's campaign isn't to take the presidency, Sanders already proved that is impossible (never forget what they did to the Iowa caucus). It would be amazing if he won, but the real goal of my support is to build support for socialism more broadly within America so we can stop this empire from killing everyone everywhere forever.