It's at least partly because the CCP allows no competition for or critical response to its propaganda, so it's never forced to get better.

Western propaganda by comparison must run the gauntlet of free speech, critical reception, and political adversaries, and as a result has become much more subtle, sophisticated, and effective [1][2].

[1]:https://en.wikipedia.org/wiki/Manufacturing_Consent

[2]:http://www.paulgraham.com/submarine.html