Why should they support him?

Bernie Sanders is once again running for President as a democrat. As a Senator, he lists himself as an independent, mainly because the dems are too right wing for his style. never mind that they are too leftist for most thinking adults.
Sanders made the switch in 2015 when he ran against HiLlARy, but as soon as the election was over, he went back to being his old commie self.
While it is true the donks are slithering left faster than an SR-71 can fly, most of the rank and file are still believing that it is liberalism, not leftism that is the mantra of their party. That leftward shift has pushed people from the party. I left in 1980 because of Carter's inept handling of the 444 days of shame where Iran held our Embassy workers hostage.  Stayed gone as I realized that pro-life views were not welcome in donkyland.
The departure of many moderates has also had a profound effect on the republican party. Many of the people who have departed want to remain politically active, and they are beating the drum for the republican party to abandon true conservatism and shift leftwards to accommodate them.
This is harming the stability of our politics. Older conservatives are slowly passing on. Many younger conservatives are not taking an active part in party politics choosing instead to focus on jobs and family. That void is being filled by vocal liberals who seek a new home,but want to make their new home look and feel like their old home where they feel unwelcome and unwanted.
Back to Bernie. If the man wants to be a demonicRAT, he needs to disavow his prior beliefs and instead embrace and uphold the platform of the democrats. The same is true regarding Anal Oral Cross-connection.What would she and the Bern do? Go caucus with the republicans? Fat chance.
Instead the donks will expand their tent for a time, and while they fully intend to screw Bernie, the shit show will continue.
Oh, well, it makes for good theater. While the various factions vie for their turn in me too land, the real fight will be back to a couple of old white men leading the party that detests old, white, and men especially.