I think the popularity also has to do with the alienation of the western left from the USSR during the Cold War. Many of those who wanted to still adhere to a revolutionary marxist line found trotskyism ideal or convenient. Others turned to what they took to be maoism, but frankly I get the impression that most of them didn't really have a good idea of developments in China at the time.

Most people who get called ultras by ML's and related tendencies are ideologically part of the left-communist tradition coming out of Italy, Germany and Holland (there are internal differences, like italian operaismo as opposed to council communism, deeply influenced by Rosa and Pannenkoek), or anarchists. Obviously people in these traditions would not be too down with Mao on their own terms, namely because they see the chinese revolution as nationalist and the CPC as stalinist in nature, structure, etc.; but it's also undeniably due to propaganda either explicitly or by osmosis.

During the height of maoist influence in the 60s and 70s there were deffo accusations by orthodox MLs that the new groups proliferating in the former tendency were ultras. I think there are many orthodox marxists, including leninists, who see many of the Maoist period's and Mao's policies as ultra.The maosit response to them was that the soviets were revisionist, and so on and so forth, sniff :zizek-ok: