The most obvious answer is that the Sanders movement has somehow not expanded since 2016.
@berbatrick analysis is spot on.
The only thing I can add is the Bernie campaign is clearly a moral and ethical appeal for socialism in the vein of say christian socialism.Which comes with both some positives and negatives, the positives being it can put across ideas in a very simple and powerful manner but the negatives being it leaves you totally confused on the current state of things, one minute you're saying ''we need a political revolution'' and then 2 minutes later.......''My platform isn't radical change'', rather than ideas coming out of people material conditions, they are just arguments that can used to convince anyone.
The fixation on ideas and arguments(Basically of left wing version of the West Wing)leads to thinking simply talking about mass movements, somehow means they will just appear in front of us(The whole theory of the Sander campaign was to basically give him the biggest loud speaker on the planet - the presidency of the United States). And then in policy terms this type of socialism limits the ability of its own future, compare the Jobs For All program of Bernie to the Labour Party 4 Day Working Week, the full employment policy comes out of protestant view of a proud workerism,. The issue for the Sanders campaign isn't waged labour itself being awful but that people are having to work a number of jobs to survive, where as Labour hated the idea of work(Corbyn was promising a bank holiday for any old shit). Plus current polling shows that people are confident in the american economy, now that looks like it could very well change but until it does, Bernie's program was also going to be very difficult to sell(Which no one has pointed out)
Also while I get the linking of the Bernie and Corbyn, we are comparing a failed general election campaign to a primary. There's more to say Corbyn was at fault(Nearly 5 years as party leader)but also the appeal of Sanders platform if he does now lose to Biden was never that popular to begin with.
Hey, while you're at it..what do you think might be the answer to this? i don't think we will get an honest answer.
I've always took the vulgar view(Out of sheer laziness) that Warren was never really a progressive(Whatever the words means today) and that she was similar to obama - basically the left convinced ourselves that this person wasn't somehow a conservative. Warren was a republican until the age of 40, a republican during the Reagan years(Which is like being a tory during thatcher).
Having said that I think her failure to support Bernie and her supporters demanding that people be nice online, is them enjoying their ideology(Technocratic liberal feminism), their reluctancy to back Bernie is them not wanting to take the step out of their ideology(Which is something the left might have to do pretty soon as well, sadly)because while we can debate what 21st century ''Democratic Socialism'' actually means, at the very least it's a view that the current system can only be improved when working class americans organise to fight against it and for Warren and her supporters that will be a painful answer to agree with.
And to put it very crudely, most of Warren support is rich white women(And men who refer to themselves as feminists), they view politics as basically a television show, where every 4 years they get to pick a new leading star and this time they are mad because the two contestants left are both old white dudes.
