Matt Yglesias moots the reasons behind America’s anti-socialist/individual tendencies. This is no illusion. America’s Left party, the Democrats, have links with the Centrist Democrat International. This is an organization which roughly represents the international Center-Right, e.g., the Christian Democratic parties of Europe. The Democrats used to have observer status when this organization was more explicitly termed the Christian Democrat International. The point being that dirigiste and One Nation tendencies are much more common among Right parties than classical liberalism (in Germany the free market party is the FDP, which is mildly libertarian in orientation and traditionally represents the Protestant anti-clerical bourgeois). The mapping between European Left and Right is not perfect because on many social issues, such as abortion, speech, and ethnicity & race, Europeans are arguably much more conservative than Americans (remember white nationalists parties such as Vlaams Belang regularly get well above 10% of the vote, and lack of judicial supremacy results in incremental changes on social issues like abortion). But when in Europe when it comes to the market even the right-wing ordoliberals are probably most analogous to moderate Democrats, not economic conservatives.
But back to the original point, why is America exceptional? Matt alludes to one major hypothesis: that socialism flourishes in societies where class is the primary independent variable. Think of Sweden in the early 20th century, a homogeneous white Lutheran Swedish speaking society (and one where the nobility was relatively small as well and the monarchy had a history of populism back to the Vasas). The argument is that America, and in particular the American South, have been riven by racial tension to the point where redistributionist policies run up against the block of the color line.* This implies that states where the populace is relatively homogeneous, such as Minnesota, would be on the forefront of social experimentation because of fewer cross-linkages cutting across class which might slow action due to perceived disparate impacts and interests.
I have another idea. How about size? The United States is very big, and operationally has never had a servile peasant population. Unlike European nations the United States never pushed up against the Malthusian trap before the demographic transition began. The Populist Party was I think a step in this direction, and I think it is no coincidence that it arose just as Frederick Jackson Turner declared that the frontier had closed. With the rise of a post-agricultural economy just as the agrarian populist movements began to crystallize the moment was lost, and so perhaps the contingent confluence of parameters necessary for the deep well of class resentment never built up in the United States (remember that many of the urban workers who created the industrial economy were immigrants from Europe who might not immediately have been able to ally with the native working classes transitioning from the farm to the factory).
Note: To some extent I stole this idea from Philip Jenkins, a religious scholar who suggests that American’s size combined with its population’s mobility makes the prefab civil society which Protestant denominations offer more important than in Europe. This might then explain why Americans are both much more pluralistic and religious than Europeans; small dense European polities require less religious social capital to tie together localities because people already have much in common.
* It is important to remember that prior to the fixation on sectionalism, the American South was a redoubt of religious liberalism. Thomas Jefferson and James Madison are well known, but John C. Calhoun, the great senator from South Carolina, was a Unitarian. Calhoun and John Quincy Adams were in fact both Unitarians while they served as Vice President and President respectively.