Complex sentence: As late as the early 1960s, there were very few African-American members of the Screen Actors Guild, since there was a lack of opportunity for them as black performers were confined mostly to race films until the 1960s.
Split into simpler sentences: As late as the early 1960s, there were very few African-American members of the Screen Actors Guild. Since there was a lack of opportunity for them as black performers, they were confined mostly to race films until the 1960s.