Answer:
Changes abound throughout the 20th century. Einstein's claim that space and time are relative transformed how people saw reality. These two thinkers inspired concepts about the subconscious and symbolism in everyday life. Airplanes and vehicles sped up travel and exposed people to new areas, civilizations, and ways of life.
Racial, socioeconomic, and gender equality movements transformed conceptual and political landscapes. The realities of war were brought home during WWI. Hitler and his army perpetrated unspeakable crimes throughout WWII, shattering any utopian worldviews. Genocide and violence were no longer isolated incidents. Gritty reality is one way this showed itself in drama. Some of the greatest playwrights of all time wrote about ordinary people doing ordinary things with a critical eye.
Unlike some past theatrical idealism, these plays tried to show the broken structures and false ideals of current society. These plays generally depicted subtle human emotion and character dynamics. Unlike some of the previous playfully bawdy theatre, this new realism was keen on portraying the world's unpleasant truths.