Conservatives already do call antifa communists. You apparently just don't spend a lot of your time consuming right wing propaganda. If you google "communist antifa" (or for this purpose, even better if you use a shitty engine like Bing, since google does more to manipulate their results) and scroll past the Wikipedia entries, you see MSM articles pushing back against the idea that antifa is a communist front. If you go a page past that, you start to see tons of articles and blog posts pushing the idea that antifa were historically a 5th column out to overthrow liberal democracy, were funded by east-german and soviet secret police, etc.

Hell, even WaPo was publishing editorials pushing this idea as far back as 3 years ago.

Antifa members are the violent advocates of a murderous ideology that, according to “The Black Book of Communism,” killed between 85 million and 100 million people last century.

They're never not calling leftists communists. I suspect that American leftists and liberals have just kind of learned to tune it out.