My apologies for stepping in Matt Holt's territory, but I was preparing this quick post before Ezra's intervention (which makes it even more relevant), so I'd thought I'd go ahead anyway. I wanted to strongly recommend Antonia Maioni's superb book Parting at the Crossroads, which is a comparative study of the emergence of different health care policies in the United States and Canada. To put it in Brad Plumer's terms, Maioni (like Brad) emphasizes the importance of structural factors, with some additional explanations based on Canadian federalism and party politics.
The Canadian case is particularly illuminating because it allows us to control for factors that are not controlled for in comparisons to Europe. Maioni's study makes it clear that three of the factors adduced by Plumer have been overrated in their importance. The strength of organized labor cannot explain the difference because at the time Canada developed its single-payer system organized labor was significantly weaker in Canada than in the U.S. The opposition of interest groups also doesn't give you much leverage, since the Canadian Medical Association was just as steadfastly opposed to single-payer at the time as the AMA. Finally, while path dependence explains why single-payer has endured, it cannot explain why Canada created a different system, since they started from a similar point. The key factor, then, is the political institutions. Canada's centralized Parliamentary system allowed for major changes that the veto-point laden Madisonian institutions of the United States have not. I haven't read Jill Quadagno's book, but as Plumer describes it a comparative analysis makes clear that she left out the most important factor. As Sven Steinmo titled his article about the failure of Clinton's attempted reform, "it's the institutions, stupid."
As a final note, another interesting aspect of Maioni's book is that the Parliamentary system causes federalism to have somewhat different effects than in the U.S. Federalism can mitigate major reform, in that lots of policies are left with a large number of subgovernments , but it can also facilitate innovation. In the Canadian system, where it's far easier for the federal government to pass major legislation, these innovations have a greater chance of becoming national policy than in the U.S. And this is what happened in Canada. Socialized medicine was first created in Saskatchewan by the government of Tommy Douglas (whose party was the CCF, the forerunner of the modern moderately socialist NDP.) The CCF/NDP used its regional base to get enough seats to exert some influence on national government, as the Liberals wanted to co-opt them. Once the Liberals decided they wanted to do it, the relative autonomy of the Canadian state allowed them to do it despite interest group opposition. Unfortunately, this can't really be replicated in the United States. Until a rare opportunity like 1937 or 1965 comes along, I'm not sure any strategy can make major health care reform viable.