Cubism, Futurism, and Constructivism

J.M Nash stated that Cubism, Futurism and Constructivism were the three most important movements in early twentieth-century art. While they all fell under the umbrella of "modern art," they still opposed each other strongly.
The birth of Cubism, according to Nash, was private. It is interesting that while we see Cubism as a completely revolutionary form of art, it really was not conceived as a movement because it was not public. This is almost a counterintuitive idea because to us, the general public, Cubism (especially that of Picasso) truly exhibits the departure from the concept of "art" to what we see as "modern art." There were two major Cubists, Picasso and Braque, who played off of each other to develop their arts, but it is puzzling to note that while Picasso is so well-known, Braque is not. What about Picasso's paintings that made him stand-out so much in the course of art history?
Nash's discussion on Demoiselles d'Avignon by Picasso brings about an interesting idea that initial reactions to the painting were mainly disgust, uneasiness and shock because of its "newness" which translated to "ugliness." People who have been conditioned to view traditional art, such as Renaissance art, as "beautiful" were not used to this new type of art; the "Medusa-masks" in Demoiselles d'Avignon were simply hideous because they screamed primitive, grotesque art. This idea of rejecting things that we are not used to as simply "gross" or "weird" seems to be true not only in art, but in other aspects of life, like how some people react to same-sex marriage. This fear and uneasiness with being confronted with new ideas are a natural response programmed into human brains. But why did Picasso choose to paint in a style that is characterized as bold, harsh, distorted and primitive? What values did he see in this, or in other words, what was he really trying to do? Speaking of primitive, it is interesting to note that after long periods of perfecting beauty, representation the realistic, perspective in art, we go all the way back to primitive art. What contributed to this tracing-back of artistic style?
The discussions on Futurism and Constructivism were not as lucid as the one about Cubism was. Futurism seemed to be mainly focused on human action, violence and anarchism as a response to WWI, which is well illustrated by Marinetti's Manifesto of Futurism. This almost seems like "scary" art, or even pessimistic, which some people argue is realistic. Why it is called Futurism, I am still not entirely certain.
Overall, I get the impression that the three movements were a bold step away from traditional art, which conveyed beauty in ways people were generally comfortable with (for example, even though Impressionism was criticized initially, people didn't see it as something grotesque or obscene) and completely threw people off balance, forcing them to confront new ideas that made some uncomfortable.