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Chrisy1 Well, there are a few things working against Bernie. For one thing, his policy positions are very similar to Warrens', which splits the social democrats between him and her. Biden is kinda the main corporate democrat shill game in town, so he doesn't suffer from this problem as much.
Generally, Bernie has a lot of enemies in high places. CNN and MSN constantly smear him and his policies because he's farther left than their little darlings and more cynically, because his positions would defund the Hell out of the industries that are probably bankrolling the DNC and these media outlets. It's harder to do well in polls when major "respected" media outlets are calling your super normal plans fantasies and doctoring photos of you (true story!) day in and day out.
Bernie Sanders also suffers from a perceived lack of electibility; lots of people think Biden is more likely to win the general election that Sanders is (cough cough Hillary Clinton) and nobody wants to nominate a guy who's gonna lose.
Other than that, Bernie is kind of just... a lot. It's one thing to vote for a guy who's gonna do a UBI or a guy who's gonna forgive student loan debt; you can debate those issues and largely win. But Bernie is a guy who wants to do universal healthcare, free college, guaranteed jobs, a wealth tax, reach 100% renewable power, universal housing, and on and on ad infinitum.
Because humans are wired with a bias towards fear and doubt, Bernie Sanders is always gonna have a tough time winning. Because even if you agree with two or three of those proposals, you really have to agree with ALL of them or at least most to be able to vote with great confidence. Because even one horrible policy could drive the country into the ground, at least in the eyes of the public.
His strategy is very bold and very aggressive, but that's also part of what makes him so damn strong. He has more enthusiasm among his base than any other candidate and that just might get 'im over the hump.