The short basic answer is fear of state control, of which communism is the most extreme example.
And socialism is seen as a gateway to communism yep!
The longer answer is hugely complex and relates to
How America was created - essentially it's foundation is a country that was created on the premise of obtaining personal freedom - particularly relates to the freedom from state control as aforementioned but exemplified in freedom to practice any religion as so many of the first settlers were fleeing religious persecution.
Anti taxation - Boston tea party, independence war etc, no taxation without representation etc
The immigrant experience - immigrants welcomed but expected to forge their own way through hard work with little to no state assistance
The American dream - which even Americans can't agree totally on the meaning!
The pioneer experience - survival of the fittest more extreme here than in many other countries in relatively "modern" history, you have to remember that "settling the west" only happened a few generations ago, there are still towns even whole counties that are little changed from this time period. Every man for himself
Capitalism - success measured almost purely in terms of material acquisition, ostentatious displays of wealth, rags to riches, failure measured in terms of lack of assets, little to no positive meaning attached to psychological or emotional achievement, even their focus on academic achievement or sporting achievement is measured by how that translates to material acquisition OR preventing others from being successful and therefore making money eg space race