I think these ideas are pretty standard but worth thinking about.

One line of thought is that the unique economic development pattern of the US -- relative prosperity  (even pre-independence) by world standards (immigrants were always leaving Europe to get here, never the other way around) due to abundant land and natural resources and a systematic labor shortage -- could satisfy the material needs of most Americans without resort to radical politics.

There is also the "exceptionalism" of American political development -- in the US, political equality (universal white male suffrage, free public education) preceded industrialization, with its concentrations of wealth and economic power. In Europe movements for political equality came later, and coincided with industrialization, thereby more firmly linking together issues of political and economic equality.

Thus socialism has never been on the agenda here, organized labor has tended to be focused on "bread and butter" issues and not on reshaping society (more radical 19th century labor organizations like the National Labor Union or the Knights of Labor came to nothing), and the state is much more firmly in the hands of capitalists.

Even when the American left was most firmly in power in the 1930s to 1960s, it was a pretty conservative liberalism, as suggested by British journalist-turned-historian Godfrey Hodgson, who summarized the ideas of post-WW2 US liberals in what he called an "ideology of the 'liberal consensus'":

1. The American free-enterprise system is different from the old capitalism. It is democratic. It creates abundance. It has a revolutionary potential for social justice.

2. The key to this potential is production - specifically, increased production, or economic growth. This makes it possible to meet people's needs out of incremental resources. Social conflict over resources between classes (what Marx called "the locomotive of history") therefore becomes obsolete and unnecessary.

3. Thus there is a natural harmony of interests in society. American society is getting more equal. It is in process of abolishing, may even have abolished, social class. Capitalists are being superseded by managers. The workers are becoming members of the middle class.

4. Social problems can be solved like industrial problems - the problem is first identified; programs are designed to solve it, by government enlightened by social science. Money and other resources - such as trained people - are then applied to the problem as "inputs." The outputs are predictable - the problem will be solved.

5. The main threat to this beneficent system comes from the deluded adherents of Marxism. The United States and its' allies, the Free World, must therefore expect a prolonged struggle against communism.

6. Quite apart from the threat of Communism, it is the duty and destiny of the United States to bring the good tidings of the free-enterprise system to the rest of the world.

Remember, this is what liberals (i.e. what passes for a left wing over here) were thinking.

American liberalism became more a theory of technocratic social and economic management than a political movement after World War II, typified by JFKs 1962 Yale commencement address:

Today...the central domestic problems of our time are more subtle and less simple. They do not relate to basic clashes of philosophy and ideology, but to ways and means of recasting common goals--to research for sophisticated solutions to complex and obstinate issues.

What is at stake in our economic decisions today is not some grand warfare of rival ideologies which will sweep the country with passion but the practical management of a modern economy. What we need are not labels and cliches but more basic discussion of the sophisticated and technical questions involved in keeping a great economic machinery moving ahead. ...[T]he problems of...the Sixties as opposed to the kinds of problems we faced in the Thirties demand subtle challenges for which technical answers--not political answers--must be provided.

Anything more radical than this - Debsian socialism, the IWW, Populism, Wallacite Progressivism, Martin Luther King's attempt to expand the civil rights movement to consider issues of economic inequality - tends to get either co-opted or cut off at the knees in the US.