Maybe I misunderstand your comment, but my point is they didn’t. Communism/socialism is red, and the US is blue+red. If the “left” side of US politics feel a need to distance themselves from socialism, they need to claim blue as their main color. No one is likely to mistake Republicans for communists so it’s safe for them to use red.
Another way to put the same argument is that Democrats are centrists/liberals rather than left wing/socialists, so the red used by actual left-wing parties in other countries just doesn’t fit.
Apologies, I meant to say why would Republicans pick red in that case, since they're even more anti-communist, but forgot to be specific enough in expressing that
Another way to put the same argument is that Democrats are centrists/liberals rather than left wing/socialists, so the red used by actual left-wing parties in other countries just doesn’t fit.