It’s basically due to the fact that the socialist tradition in America was A. historically (mostly) confined to European immigrant enclaves in the Northeast and Midwest and then B. was viciously repressed in the first and second Red Scares, stamping out what socialist tradition had managed to form. (To give an example of the chilling effect of the first Red Scare: in 1912, Eugene V. Debs got 900,000 votes or 6% of the total in the general election as the Socialist candidate; by 1924, the Socialist candidate got less than 30,000 votes or .13% of the total.)
The result was, that unless you came from particular immigrant or ethnic backgrounds and/or were a “red diaper baby,” most Americans post-1945 especially would have no exposure whatsoever to the mainstream socialist traditions and their basic political vocabularies that would have been entirely normal to anyone who grew up in a working class European family in the same period.
Add onto that a rich and vigorous history of various conservative forces in the U.S labeling pretty much any domestic liberal reform and more than a few soon-to-be-overthrown foreign nationalist governments as “Communistic,” and you begin to see why our grasp on the political language of socialism is so feeble.
I’m a bit confused by this conclusion. The First Red Scare had begun in 1917 and was over by 1920. On Election Day in November 1920, Debs earned 900,000 votes, slightly more than he received in the 1912 election by raw vote totals. In the 1916 election, before the First Red Scare, Allan Benson ran as the Socialist Party’s candidate and received 3.19% of the popular vote. In the 1920 election, Eugene Debs earned about 3.41% of the popular vote, slightly higher percentage-wise than the previous election but well within standard deviations, and much higher than expected if the Red Scare was indeed suppressing the movement. If the First Red Scare was responsible, shouldn’t the drop off have happened earlier, between the 1916 and 1920 elections when it was in full swing? Certainly not in the next four year period well after the Scare was already over and discredited.
The First Red Scare was not over by 1920: you have the Palmer Raids in January, Sacco and Vanzetti’s arrest (their trial would run through 1921 although the cause celebre would continue through to ‘27) and the expulsion of the New York Socialist legislators in April; the May Day scare; Debs was still in prison until late December ‘21, which is the date that I’d pick as the end date, personally.
Debs’ 1920 total was impressive given that he was running from prison, but it was definitely down from 1912 as a percentage of the vote and given the dramatic expansion of the electorate due to the passage of the 19th amendment (26.7 million in 1920 vs. 15 million in 1912). Regardless, I would argue that the before and after picture with 1924 is the more instructive.
But pedanticism about what date from the 1920s we’re going with aside, the first Red Scare had a lasting impact on the development (or lack thereof) of an American socialist movement/tradition and (cumulatively with the second Red Scare as I mention in the OP) thus influenced how many Americans born after ‘45 would have ever been exposed to socialist ideas and concepts.