What is the tl;dr of 20th century art theory?

Answer 1

The 20th century saw the Industrial Revolution, which changed the standard of living for many people in the West. This new widespread automation of work culture allowed art to branch into expression and exploration of the personal, instead of its previous role as a form of skilled labor that was marketed to the richest.

It's in the 20th century that scholars begin to see artistic movements where the ideals and theories reacted to current events (like Modernism, Dada, and Surrealism,) as well as the artist's own life (like Expressionism, Impressionism, and Fauvism) really gain momentum. There were art movements dedicated to venting social attitudes about the political upheaval of both World Wars, art movements for exploring the burgeoning sciences of psychiatry and sociology, and art movements for publicizing the standard of everyday living among people who were not rich and famous.

In a lot of ways the art of the 20th century is far more dependent on social issues than previous artistic periods, but that dependency became a strength of vision and a tool for expression rather than holding back artist from developing new styles.