Why did post-war Italian directors cast factory workers instead of stars?

Partly because they had little choice, and partly as a principle. When Roberto Rossellini began shooting Rome, Open City in January 1945, the Cinecittà studios were damaged and requisitioned by Allied forces to house displaced persons, and the film's style developed from circumstance. Neorealist films were shot on location, frequently with non-professional actors, and told the everyday stories of poor and working-class people.

Vittorio De Sica's Bicycle Thieves (1948) shows the principle. No major studio would back it, so De Sica raised the money from friends, and, following the precepts of neorealism, he cast only untrained actors. Lamberto Maggiorani, a factory worker, was cast after he brought his young son to an audition, and 8-year-old Enzo Staiola was noticed watching the shoot in the street while helping his father sell flowers.

Rossellini's film was short of everything. When the production ran out of film, an American soldier supplied short-ends and rolls that the Signal Corps threw away because they might be fogged or scratched. Rossellini hired mostly non-professional actors and relied more on improvisation than on a script.

Abroad, the results were celebrated: Rome, Open City won the Palme d'Or at Cannes in 1946, and a 1952 Sight & Sound poll of filmmakers and critics deemed Bicycle Thieves the greatest film of all time. In Italy, though, Bicycle Thieves was viewed with hostility as portraying Italians negatively.