Over the many years and many conversations, I've never really heard Professional Democrats discuss why this is or what can be done about it. Leaving aside complaints of bias or particular instances of media malpractice, a fundamental problem is that conservatives simply set the agenda. Even if what they're mad about is something dumb-even-for-them, it still gets covered.
Once upon a time there was a fairly simple explanation for this, or at least description of how it worked, and that was that Matt Drudge Rules Their World. That's no longer true, but nothing much has changed.
I have *my* explanations, but to some extent the explanations don't even matter. Instead: what are you going to do about it?