The communists are too strong, too powerful, too invincible. Americans know this deep down inside, and that's why they are too scared and too terrified to take on the communists. Why start a fight you can't finish? Americans would be dumb to not be scared.
The communists wear body armor that makes them invincible to bullets. They're also armed with weapons much much much stronger than the guns that Americans have. Communists have NUCLEAR FUCKING WEAPONS for crying out loud. But they also have drones, artillery and firebombs that they can resort to without radiating land.
A civil war between current Americans and the communist government would be a 10/10 curbstomp with all Americans getting blown to pieces, it would be the quickest and most humiliating civil war in human history. Americans trying to stupidly and selfishly take on their government would be the equivalent of a bunch of jackals trying to take on a lion... they'll all get obliterated.