and the reason is that this piece of political propaganda and the kind similar to it was rare in China, not because they were banned, but because the era it made was short:
The era was from the end of the Cultural Revolution in 1976 to the beginning of the reform in 1980. During this short era, China was ruled by a bunch of hard line conservative commies who denounced and hated the Gang of Four and the Cultural Revolution, but still adores Mao, and hence making excuses for Mao and the Cultural Revolution he launched by putting all of the blames on the Gang of Four.
This movie is the political propaganda that accurately reflects such behavior/mentality of these hard line conservative commies. (And thank God, they did not rule China for too long).