The full quote: "And anti-communism was genuinely and viscerally popular in a country built on individualism and private enterprise where the nation itself was defined in exclusively ideological terms ('Americanism') which could be virtually defined as the polar opposite of communism."
Hobsbawm: The Age of Extremes, Abacus, 1995, Page 235.
What he means is that US-culture is built on ideas that run contrary to socialist ideology. Liberalism played such an important role in the genesis of the US nation, unlike any other country, even France, and is deeply engrained in the culture. The US nation isn't built on ethnicity and heritage but on the political ideas of Liberalism. Things like the "American dream" or the "City upon a hill topos" are deeply intertwined with American culture, and all of those run contrary to socialist ideology. Therefore it was relatively easy for anti-communism to spread as the foundation for it was already layed long ago.