Welsh pretty much nails it. Not just why progressive bloggers failed but why progressivism in general is failing so abysmally.
[P]rogressives have no power, because they have no principles: they cannot be expected to actually vote for the most progressive candidate, to successfully primary candidates, to care about policy first and identity second, to not take scraps from the table and sell out other progressive’s interests.
The Tea Party, say what you will about them, gets a great deal of obeisance from Republicans for one simple reason: they will primary you if they don’t like how you’ve been voting, and they’ll probably win that primary. They are feared. Progressives are not feared, because they do not believe enough in their ostensible principles to act on them in an effective fashion.