Why were the socialists and communists useless during the Great Depression?
In the United States, the socialists and communists were "useless" because they had no chance to be "useful." Although it is doubtful that they would have been able to get the US out of the Depression, they did not have the chance.
The United States never turned to radical politics in any major way during the Depression. This happened, I would argue, for two reasons. First, the US has a deep commitment to democracy and freedom, both personal and economic. Americans as a whole tend to have an aversion to communism and the sort of government control that comes with it. Therefore, Americans were unlikely to give the communists and socialists a chance to be relevant.
Second, President Roosevelt's actions effectively precluded radicalization. FDR acted to try to mitigate the problems that had been caused by the capitalist system of the time. This acted as something of a "pressure release" to let off some of the anger that people had. If a president had come to power in 1933 and tried to enact laissez-faire policies, anger could have erupted and given socialism a chance. But FDR's New Deal let some of that steam off and eased the pressure that might have pushed towards more radical solutions.