The USA is built on capitalism. In the 19th century, immigrants from across the world flocked to the US for the promise of a better life, that they themselves would forge. The national theme of the later 19th century was Manifest Destiny: the idea that independent-thinking and enterprising Americans could and would spread across the continent. The theme of the later 20th century was "communism is scary!", which is not the best way to promote a socialist state.
In short, the USA is so deeply capitalist that it's doomed to remain that way until it falls apart.

Although, as my history teacher pointed out, socialism/communism has been practiced on US land, by the Native Americans residing there.