Tbf, in 2016 the DNC and Clinton were so arrogant they literally said, "we don't need your votes." Like every political campaign all they needed to do was make some empty promises to try to get them on board.
In my experience, the difference between die hard Sanders supporters and other leftists or liberals is that they tend to believe that the on going class war is the single biggest issue we face in the US today. Everything else is a distraction to keep that otherwise overwhelming block of the population segmented into competing blocks of people. The Dems just foster an attitude of "competing but inter-supportive" and the GOP has one of "competing, and you should enslave your enemies." The latter of course only really appealing to white conservatives and useful idiots.