What "hard left" candidate? Bernie isn't "hard left". Warren isn't "hard left".
A better question to ask would be: why are mainstream / safe / "acceptable" Democratic candidates shifting more and more towards the right. Republicans are happily dragging the Overton window into the sunset with the Democrats cautiously following under the notion of "bipartisanship" (and not the reality that is most high-ranking Democrats, and their donor base, are perfectly happy being just as bad as the Republicans if they think they can get away with it).
There are a lot of valid questions that could be asked of the DNC, but no, instead we get these silly generalisations about what this subforum would "approve" of. A generalisation, heh. Ironic that. Always weird how they seem to be okay when targeting anyone left-of-centre