Apologies that this is very simplistic and reductionist: My impression is that most of the people in these movements were young and urban dwelling (e.g. Godard's film "Le Chionese"), and Maoism speaks of the revolution being agriculture based with peasants in the countryside. Considering that Stalinism is urban-based and deals with the working class accelerating the revolution through industrialization, wouldn't this have been more aligned with many of the ideas the New Left espoused?
They were familiar with the horrors of Stalinism, and not yet familiar with the horrors of Maoism.
The disaffection of European intellectuals with Stalinism starts in the 1930s-- the purges, the show trials, the murder of Trotsky, the Molotov-Ribbentrop non aggression pact. During the Second World War, opinion of the Soviets rebounds a bit . . . but Kruschchev's "Secret Speech" denouncing Stalin's Crimes ("On the Cult of Personality and Its Consequences") of 1956; the crushing of the Hungarian uprising in 1956, just what the breaking point was for any particular western communist might be varied.
Some parted ways with Moscow early, though many including the Parti communiste français stood by them till the bitter end-- but it's safe to say that there's a steady attrition of enthusiasm among western leftists for Moscow, due to the troubling fact that they were running a very unpleasant police state.
Of China, most people knew very little. Start with the language barrier, and then consider how little the Chinese cared about communicating with the West. People with lefty fantasies were left free to fantasize about China, where they knew too much about the Soviet Union for the fantasy to stick. So, for example, there was a pretty good understanding of famine conditions under Stalin-- but far less awareness of Mao's famines.
So China was, by the 1960s, a more attractive substrate for naive fantasy. And Godard was hardly a political economist-- his "attraction" to Mao was based on superficial aesthetics, far removed from facts or the practicalities of political economy.
Godard urged his interviewers to accept two principles. The first is the definition of a militant film as one in which "the screen is a blackboard." Didactic by nature and film essay in form, the militant film, in reconstituting the problems of society, may seem as jumbled to the moviegoer as a blackboard after a complicated lecture. Because Godard bombards the screen with seemingly unconnected images and multiple sounds, his work seems to be the cinematic equivalent of Finnegans Wake - "quashed quotatoes, messes of mottage" (183.22). Although his film seems to be a wall, not a window, to understanding, it is up to the moviegoer to respond to the film as to a lecture and to analyze with the film maker the material on screen.
With a second axiom Godard advised his opponents about workers for whom the militant film is made. In "Culture and Art," section xxxiii of Quotations from Chairman Mao, Mao says, "All our literature and art are for the masses of people, and in the first place for the workers, peasants, and soldiers and are for their use." Interpreting this pious generality, Godard says that his film is not made for the masses but from them and through them. He is not making films to be marketed, like Bondarchuk's War and Peace, which, according to Godard, is no more Marxist-Leninist than Gone with the Wind, and which sustains middle-class society by its received rhetoric. The worker attends See You at Mao for instruction. Ideally, the film leads to discussion, possibly in the gardens outside the factory
I think that gives you a flavor for the level of analysis Godard was working with-- aesthetic and idiosyncratic, rather than factual or rigorous.
Finally, remember that substantial numbers of Chinese workers came to France during the First World War and later a number of future communist party leaders like Zhou Enlai and Deng Xiaoping followed; France had at least some feeling of cultural affinity, mirage though it might be.
Sources:
Darkness at Noon - Arthur Koestler's 1940 novel was one of the first Western intellectual indictments of Stalin's police state from a former "believer"
Georges Marchais, 1920-1997: Death of a Blunt Instrument -- Marchais was the head of the old line Stalinist Communist Party in France. While Mao's China was popular for young dreamers, Marchais' Moscow oriented PCF was a far more potent political power.
Godard's Marxism A thoughtful analysis of just what sort of mumbo jumbo Godard had on his mind. Call it "revolution for poets"-- there's more inquiry into semiotics than interest in the death tolls.
THE YEARS IN FRANCE OF CCP LEADERS Not the best source on Deng, Zhou and other Chinese leaders' years in France, but it will do as an introduction.
French Maoism Very good general survey of what French Maoism was about, by Belden Fields, a venerable American professor of political science, with a particular focus on Western European Marxism