They believe rather rely on facts. And they expect support without earning it.
Yes, many left-wing ideas are popular in the abstract, because they are genuinely good and positive. They ARE ideas to strive for, most definitely. But the only real way to realize those ideas is to deliver concrete plans on how to deliver them. "Congress will look out the window and see millions of young voters" is NOT A CONCRETE PLAN. Lying about how much implementation will cost is not a concrete plan! Promising to raise taxes on the middle class is not a concrete plan.
Spitting in the faces of your potential allies is not a concrete plan. The America public is not as dumb as the media tries to make us believe, and they know a load of horse manure when they see a politician trying to sell it to them.
We've seen America reject two tea parties now, the right-wing and left-wing versions both. On the bright side, there are only two fringes to put us through that embarrassing process, so maybe, just maybe, they'll understand now that they have to expand their coalitions, rather than shrink them through purity tactics and litmus tests that eliminate the possibility of support from mainstream voters.
Millennials will be a major force in politics in a few years. I'm just worried they've learned the wrong lessons this cycle, that you can lie about your policies and still succeed, that you can be angry and violent and dangerous and still get votes, and that you can squander the possibility of partial progress in the name of "principles," which actually helps no one ever. Voting is not about feeling good about yourself, nor is it about demanding that a candidate "inspire" you just to do your basic civic duty of participation.
We'll see what happens over the next few years, I guess. Hopefully the passion from this year doesn't sour into permanent bitterness all because Bernie Sanders was a dissembler of massive proportion.