Art went private at the beginning of the 20th century. Back then Cubism's quest for a new visual language, abstract art's pursuit of purity of form, and Surrealism's sense of inwardness had little appeal to a public who viewed Modern Art as self-serving and difficult.
Ironically then, Modernism's avant-garde concerns led a retreat from the public arena.
It was not always so. If public art is art produced for, and often paid for by, the community, then history encompasses far more public than private art.