This is a very tough one. From very early on Socialism was always tainted in the United States by its association with foreign radicals. Events like the Haymarket Riot set the tone early on, with the American Middle class being likely to associate socialism with bomb throwing anarchists and revolution.
Though I am not sure about the exact moment Socialism was dead in the US, I think it was certainly dead as a force by the early 1950's with the Red Scare. By that time it was too closely tied to Communism in the US public mind. This was of course only possible because the Socialist Party had never managed to establish itself as a strong movement distinct from the US Communist party, but nonetheless was a political fact this time.
Earlier, Socialism's reputation in the US was eroded by its association with Pacifism during the WWI. Even though many Americans came to oppose America's participation in that conflict after the war, the socialists' opposition was often seen as treasonous. The IWW (International Workers of the World, a Socialist organization) was violent in both rhetoric and action, and probably alienated many Middle Class Americans both by its actions and its reputation.
My final answer: Socialism in the US was dead on arrival because it was associated with foreign immigrants and radicals. Because it could never be portrayed as anything other than a foreign ideology, and was so antithetical to traditional American political beliefs, it was always doomed to be a minority movement.