In one word: anti-communism.

The intense anti-communism at home led to this conformity. If you stood out, you could be labeled a communist. Thus, everyone looked and acted the same. There's a really good book about this - Elaine Tyler May's "Homeward Bound." In it, she argues that national security in the 1950s became linked with domestic stability. In other words, containment of communism abroad meant containment of dangerous social forces at home.

I would also note that this image of the conformist 1950s is just that - an image. Not everything was pleasant and happy. Betty Friedan's "The Feminine Mystique" argues that women of the 1950s were deeply unsettled and unsatisfied with their lives. Indeed, it was during the 1950s when antidepressants were invented (although many women could not access them and chose other barbiturates instead). It was also a decade of rebellion, from James Dean and Elvis Presley to the start of the Civil Rights and Women's Movement. These two versions of the 1950s coexisted uneasily with one another.