McCarthyism is probably the most succinct answer. The Cold War directly aligned us against the Soviet Union, and a key means of villainizing the USSR and its citizenry was to paint their core governing beliefs as heretical to the American way.

There is also no doubt in my mind that Socialism’s strong connection to Union activity in the US also incentivized Corporate Barons to lobby against Social Politics hard. I have not seen any specifics about that myself, though. Modern lobbying efforts are well-known, though.