In the right wing U.S., the Democratic Party is split between numerically and institutionally dominant right liberals and a large progressive minority of party members and independents - the overwhelming majority of whom habitually end up voting Democrat. I feel on safe ground asserting 60/40%, though perhaps progressives number greater than 40%.
"Sanders progressives" is shorthand for the latter. Sanders' campaign threw this split into relief, forming a kind of proto-insurgency within the Democratic Party.
Why should "any 'Sanders 'prog' ever vote D?" The general rationale is that Democrats are the 'lesser of two evils.' One does not have to accept that reasoning, but that is surely what it is...And, of course, that goes for Sanders too - who, as an independent, votes Democrat, and, in line with the 'lesser of two evil-ist' reasoning, supported HRC.
I don't feel like reiterating my oft-repeated solution to this prob, but you can find it easily enough if you look back.