Complex sentence: The problem was that the American market was so much larger and richer, and bought up the top people, especially when Hollywood came to the fore in the 1920s and produced over 80 percent of the total world output.
Split into simpler sentences: The problem was that the American market was so much larger and richer. It bought up the top talent, especially when Hollywood came to the fore in the 1920s and produced over 80 percent of the total world output.