Because a territorial/economic struggle between two empires (as, according to Thucididyes must always happen) was sold to the people as a struggle between idealogies.
"Communism/socialism" was the other-guy and thus the "bad-ideology". Captialism was "our team" and thus the "good ideology."
Its is an oddity of Americans that we just love to turn everything into a team sport with two sides. (Look at our political system for more evidence.)
But basically, conservative american politicians crafted a religion of Capitalism and their devil is socialism. And has been shown many, many times, the less people actually know about their religion and its origins, the more fervently they believe it.
Edit Addl: Its mostly the older folk in America who remember the cold war that still believe this nonsense. The better educated young folk by and large have no fear of socialism and are confused by the fear they see in their elders.