It’s just that “commies” is a stupid pejorative out of the 1950s. It sounds antiquated, convinces nobody, and only people who already hate communism will listen to you. It’s like Dems who refer to Republicans as “Rethuglicans” or Republicans who call Dems “Democraps”, or like lefties who call everyone to their right a nazi.

There’s also something about shortening “communist” to the childish “commie” that makes you sound less serious, IMO.