Why did black athletes become prominent during the 1930s and 1940s?

In 1936, Jesse Owens represented the US on its Olympic track and field team in Berlin. At the time, Germany had fallen to Hitler and Nazism. When Owens won four gold medals and broke two world records, his performance at the games undermined Hitler's assertion of a German master race—that is, the false belief that German and other Northern European people (Aryans, as he called them) were superior to other "races" of humanity.
Owens's triumphs were well-timed, for they put a chink in the armor of the Nazis' belief system and, as a representative of the United States (a country with its own white supremacist values), he was a living example of a democratic ideal.
A year later, on June 22, 1937, Joe Louis knocked out the world heavyweight champion James J. Braddock. Louis was not the first black man to become heavyweight champion (that title belongs to Jack Johnson who was the champion from 1908 to 1915), but he held the title longer than any other champion in any weight class. He lost it on March 1, 1949, due to his decision to retire.
Joe Louis was indomitable and a champion to black people who loved to see him knock down every Great White Hope—that is, a white boxer whom white supremacists hoped would defeat Louis—who entered the ring with him.
The 1930s and 1940s were also a period of more political engagement for black people, some of whom had become Socialists or Communists. A. Philip Randolph, a prominent Civil Rights leader at the time, followed his successful organization of the Sleeping Car Pullman Porters with a mass march in the 1940s, protesting against discrimination in the war industry. Black people also played a more significant role in the Second World War. For instance, in this war, black men were allowed to become fighter pilots (e.g., the Tuskegee Red Tails). In the First World War, black American pilots were allowed to fly for the French, but not for their own country. Thus, the athletes were symbolic of the ways in which black people were capable of being sources of national pride but also could demonstrate resistance against systemic discrimination.