I think Jackson Pollock work is considered modernism because  Pollock’s contribution to the art world.

Jackson Pollock (1912-1956)  Pollock founded the Abstract Expressionist movement. By the mid 1940s he was painting in a completely abstract manner, and the `drip and splash’ style for which he is best known emerged with some abruptness in 1947. Instead of using the traditional easel he affixed his canvas to the floor or the wall and poured and dripped his paint from a can. He was an alcoholic.

The influence of the Native American art is very evident in the work of Jackson Pollock. Pollock and Native artists work using a similar process; Pollock takes direct images from the unconscious mind like images from the natives’ “spirit world”; he uses a primitivist aesthetic; he becomes “part of” the painting, akin to native American sand painters, and he exhibits similarly universalized subject matter to the Native Americans. Essentially a timeless visual language is explored when linking Native American primitive art to Pollock’s modernist art.

Pollock was very influenced by the work of Picasso, and the work Guernica, after reading an article by John D. Graham entitled “Primitive Art and Picasso”.  The idea that all human beings are “wired the same way” provides the basis for the idea that they can all be affected by similar subconscious symbols and imagery. Psycho-analytical drawings of Pollock’s, as a result, contained a strange resemblance to Picasso’s.

Interesting Video with Jackson Pollock Narrating