Because the right lost the history of McCarthyism.
That, coupled with the skin color of Barack Obama, makes it as treacherous as bomb disposal to label any left-wing politician a communist. So entrenched is this that even today after documentary proof many on the left do not believe that Alger Hiss was a communist spy. Many do not believe the Rosenbergs were guilty. Most do not believe that the administrations of Franklin Roosevelt and Harry Truman were riddled with communists in high places.
Academia is particularly committed to this view of history.
The mainstream media generally follows the holy writ as issued by academia and so we have a climate in which to allege communism is virtually the same as confessing insanity.
Please note, there is no public obloquy to be expected for accusing a Republican of racism, fascism, homophobia, or misanthropy.
We lose the wars because we lose the battle to write history and we lose the battle to set the definitions.
That bears repeating.
We have also lost the battle to properly educate our children and maintain a reasonably balanced and honest news media. The Marxists and the labor unions never sleep. The termites simply eat deeper and deeper into our weakened foundations.