That's because there are only a handful of "Bernies". A party is not a monolithical block, it's the sum of it's members, and the centrists end up being in charge because they are the ones that end up representing the party at most levels. If you want to shift the balance you need leftists to run for school boards, and city halls, and build from there by starting taking over the state committees and DNC members elected by each state (which in turn control the DNC).

If even the most extreme of the extreme right managed to do it in the republican party, there's no reason why a moderate left movement couldn't do it in the democratic party - if anything I would expect it to be easier.

The democrats are still at their core a liberal party, and ultimately running a left candidate would be against their interests.

What’s really frustrating is the Dems just dont seem to have any vision of what they want. They clearly don’t want the dystopia of the Trump party, but aren’t really offering a vision of something different or a way things ought to be. (And they won’t be able to as long as they are trying to cater to workers as well as the Wall Street class at the same time.)