The more Democrats do to reduce the deficit, the easier they make it politically for Republicans to retake power, and the easier they make it fiscally for Republicans to wreck the budget when they do. So, why try?
As he notes, the Democratic health care plans that passed the House and Senate all reduced the deficit, according to the CBO. But this hasn't helped them at all. In fact, polls show that the vast majority of Americans think they increase the deficit.
This is the kind of problem that would be solved by having independent figures who care about deficit reduction step up and point out that the Democratic health care plans cut the deficit. It's kind of thing that activists who care about an issue regularly do. If you think that something needs to be done about climate change, you'll support Democrats, hate on James Inhofe, but also reserve some nice words for a green Republican like Charlie Crist, as Brad Plumer does. Because Brad Plumer cares about climate change.
This is a thing that centrist pundits who talk a lot about deficit reduction would be in excellent position to do. But it's not the kind of thing they actually do, since the point of being a centrist pundit who talks about deficit reduction isn't actually to lower the deficit. It's just to present yourself to others and yourself as a Very Serious Person who cares about the things sober-minded Americans care about and judiciously avoids the evils of partisanship. And you can't keep doing that if you're going to go over and advocate for a party that's trying to do what you want. Even if that's what you need to do to reward people who do the right thing on your professed issue, win public support for their proposals, and keep them in power.