Authoritarianism requires a boogeyman to point and shriek at. It is required to have a big bad scary evil (and RED) enemy to justify the constant erosion of democracy and civil rights.

That's pretty much it. The reasons don't really go much deeper. There are deeper threads you can pull at and more well reasoned and thought out objections to China, but those are academic. The people in power don't care/can't comprehend anything more sophisticated than "communism bad".