The problem was twofold. First, in two-party systems, the other party always adapts: It figures out ways to counteract winning messages (e.g., Cory Gardner embracing over-the-counter birth control in Colorado), or utilizes emerging issues to help steal groups from the other coalition. Because governing inherently involves picking winners and losers, there are almost always groups for the out-party to steal.
Second, progressive centrism was always going to be difficult to maintain as a dominant ideology. Over time, the more ideological factions tend to exert the strongest pull on the party. When they win, they believe that history has finally swung to their side, and when they lose, they tend to believe it was the moderates’ fault. This tendency in the Republican Party has drawn the most media attention of late, but you can see this play out within the Democratic Party from 1996 to 2008 in the progression of nominees: Clinton, Gore, Kerry, Obama.
Obama’s campaign was centered on the notion of rejecting Clinton-style centrism as too cautious and cynical. Though Obama wasn’t really a “netroots” candidate, this theme had been bubbling among the Democratic base for a few years, finding its purest expression in the successful primary campaign against Sen. Joe Lieberman in 2006 (people today forget that similar campaigns were nearly successful against Sens. Michael Bennet and Blanche Lincoln in 2010). But as I noted in 2009, it is difficult for an actual presidency founded on the rejection of Clintonism to hold that coalition together once in office (despite the fact that Obama often governed more like a progressive centrist than his supporters would have liked).