When the movie industry first came into being in the U.S., it wasn't seen as a very reputable profession. Given discrimination against immigrants and women in other more established industries, it was only natural that they were first in line to grab jobs in Hollywood. Specifically, many ”screenwriters“ in those days (I put screenwriter in quotations because it was the silent era for movies so the job consisted of writing the interstitial title cards, not dialogue) were women:

Women had been a major force in the film industry during the silent era, particularly in the area of "screenwriting." Since dialogue wasn't needed, and inter-titles were a separate discipline, screenplays were called "scenarios", with the concept of "play" devolving onto the movie itself, which commonly was called a "photoplay" in the first generations of cinema.
June Mathis, who helped make Rudolph Valentino a superstar, wrote the scenarios and screenplays for over a hundred films, and also as an "editorial director" on many other films, from the mid-Teens until 1930.
Women directors were not uncommon during the silent era (In fact, the first "feature" film was directed by a woman, back in 1896).

After sound came to the movies, however, women started to be squeezed out of the movies. Why?

The era of the Talkie launched was followed closely by The Great Depression, and several dominoes toppled into each other in succession.

At first, movie studios were not hurt by the downturn in consumer spending, as Americans sought entertainment in the movie theaters. By the end of the Hebert Hoover Administration, attendance was declining as economic conditions worsened.
The studios were forced to turn to the New York money center banks to seek capital. The banks put their own representatives on movie studios' boards of directors. The financial experts brought in to the industry by the banks reorganized the business and imposed a corporate management paradigm on the studios. This outside influence exerted a great deal of pressure towards conformity and the imposition of strict hierarchies.
It is a truism of organizational theory that the more complex the structure, the more control is exerted over all aspects of the organization, and the more conformity is demanded from organizational players. The corporate hierarchies were dominated by men, and the pressure for conformity made the vertical, publicly traded studios inhospitable to women, who by their very gender, could not conform to the dominant corporate paradigm.

Notably, it was a woman, Frances Marion, who was the highest paid screenwriter in Hollywood from 1916 through 1935.

I think of this story when I read about discrimination in other fields. The life cycle of discrimination often repeats itself across industries, leading its victims to be early adopters of new and not yet socially respected professions.