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.