There are several reasons.

1. No major American labor union has ever tried to establish a national Labor Party:

Mainstream labor organizations in the United States, particularly the American Federation of Labor, focused largely on organizing skilled tradesmen rather than unskilled workers, and were able to plug themselves in rather effectively into the political machines of various cities and states. In many cases they, along with the Knights of Labor, managed to establish Working Men's parties in many localities, and Labor parties in many states (Minnesota's Farmer-Labor Party, now the Democratic-Farm-Labor Party, is the best example I can think of) but it never became a national model.

2. American workers came to view themselves, and their sense of class, differently:

European workers could claim a collective memory of feudalism and resentment of old social hierarchies that didn't simply disappear with the advent of industrialization. In the United States, a constant stream of immigration, fueled by westward expansion, and the perception that there was a seemingly limitless potential for economic growth and social mobility from the early 19th century up until the late 20th, led to a very different perception of class. People didn't see their economic status as irreparable, or at least, the didn't want to see it that way. The fact that in the UK people still refer to "The Class System" as a thing speaks to this. No such classification exists in popular American politics.

3. Anti-Communism, anti-socialism, anti-anarchism, take your pick...:

One cannot underestimate the power of anti-communist sentiment in the United States. There has certainly been a space created, through much hard work and sacrifice, for labor politics in the formal sphere of American politics, but no matter how far labor has advanced it has always been shadowed by red-baiting. It is a difficult enough problem for mainstream unions and political organizations, but it was a dangerous and often fatal confrontation for radical unions and political parties. American labor leaders were historically terrified of being perceived as communist, i.e. the product of foreign elements. EDIT: I should have finished this by saying "Not forming a seperate political party kept them within the confines of the "acceptable" space standard two-party discourse.

For further reading, and probably a better answer:

American Workers, American Unions: The Twentieth Century by Robert Zieger

Strike! by Jeremy Brecher

Both of these are good primers for US Labor History, and they cover this subject pretty thoroughly.

The Socialist Party: Eugene V. Debs and the Radical Politics of the American Working Class by Carolyn Skahill

Covers what is largely regarded as the most notable attempt to actually create a working class party.

Workingmen's Democracy: The Knights of Labor and American Politics by Leon Fink

Covers several examples of Knights of Labor organizing in various cities in the US, their varying tactics (from direct action to political party engagement) and the degrees of failure and success.

Whilst not answering your question, and brushing quite close to the 20 year rule, the Labour Party in the UK only ceased to be officially socialist quite recently. The commitment to nationalisation of industry was only abolished in 1994, and it was certainly running on a socialist platform in the early 1980s.