There are a number of reasons for this. I will mention three of the most important.
- The Cold War and McCarthyism. To some extent people were afraid to be different. Being different might get a person singled out and suspected of being in some way a communist or other subversive.
- Culture of large companies. The US economy came to be dominated by large companies with many white collar employees. These employees needed to act in the correct corporate way if they wanted to advance in the company. A person who acted too different might not get ahead, so people tended to conform.
- The impact of the Depression and the war. People like my grandparents (born 1911 and 1917) had lived all their adult lives in times of crisis. By the 1950s, they just wanted to have stable lives with security for themselves and their families. This helped lead to more sober and conformist behavior.