I mean, what'd you expect? The so-called "progressive" movement had four years to make in roads with the rest of the Democratic Party, and they just didn't put the work in. Instead, they carefully crafted an outsider ecosystem built on online activism and a handful of news aggregators and opinion writers. All well and good if you have the numbers to win, but they didn't. So, when it came time to run again the Sanders campaign was filled to the brim with bomb throwers and outsiders who didn't play well with others. The result is they've managed to turn off leftists who have further concerns beyond their economic policies. Sanders' plans are all well and good, but they never actually address, for instance, my concerns, but mainline Democrats do.
It was a pretty easy primary vote for me in the end. I'm not interested in smashing the Democratic Party. It's the only vehicle that's shown any effectiveness in dealing with my concerns.