Why? The US were were the last major country to recognise the USSR (in late 1933, just before the USSR joined the LoN), and the USSR's various stated purposes (collectivisation, state atheism, …) were very much polar opposites to the US's.
The alliance was against Nazi Germany, not for anything.
The large fascism homefront in the US consisted of the industry, the press (William Hearst) and the banks, the government was largely fascist (after the business plot and Roosevelt's death), and it took a lot of (british) efforts and some sunken ships to turn the public around, against the Nazis.
More here: https://www.globalresearch.ca/a-brief-history-of-fascism-in-...
The claim that the US was "largely fascist" after WW2 also seems... questionable.
Well, the US style of fascism after WW2 can of course not be called as such in the homeland. The regimes it installed over the world were purely fascist military dicatorships, inside the country it was a modernized version of fascism, commonly called cooperatism. But since the 70ies scandals worldwide european lefts simply call it fascism again. Nothing changed since the 30ies.