Why did consumer culture grip the United States after World War II?
First, we must realize that we cannot know for certain why changes in culture happen. We can guess, but we cannot know. That said, we can conjecture that consumer culture gripped the US in this time as a reaction to World War II and the Great Depression.
The Great Depression hit the United States in 1929. World War II then followed. This meant that, in the post war years, Americans had gone about 20 years since they had been prosperous and able to spend money. In the Depression, most people did not have the money to spend. During the war, there was plenty of money, but men were off fighting and rationing meant there was nothing to spend it on.
By the post-war years, America had money and it had things on which to spend the money. People were also emotionally and psychologically ready to spend since they had finally gotten out of the crises of the 1930s and 1940s. They were ready to enjoy life and they now had the means to do so.
We can conjecture, then, that these factors allowed the consumer culture to grip the US in the post-war years.