The Communist Party in the United States never attracted anything like the levels of mass support of their European counterparts. To a large extent, this was because the New Deal under FDR ameliorated the worst conditions of the Great Depression and so there was a real alternative for those disillusioned with the workings of unfettered capitalism.
Nonetheless, for many Americans, the New Deal didn't go far enough. They felt that unemployment was still way too high, continuing to cause significant misery and suffering to millions. Reformed capitalism, however benign, was still capitalism, and therefore prone to the same ills as its free-market variant. No matter how much the Roosevelt Administration intervened in the running of the economy, there was nothing to stop a recurrence of the same kind of problems that had led to the Great Depression in the first place.
Many of those who felt this way embraced the communist message. The communists didn't want to reform capitalism as FDR did; they wanted to abolish it altogether. They fervently believed that this was the only way to bring about a more equal society, in which the social evils of want, idleness, and ignorance were defeated once and for all. Communism seemed to offer hope for those, especially among the poorest members of society, for whom the New Deal did not go far enough in transforming life chances.