First, we have to realize that it is not possible to say with certainty why political attitudes change when they do. All we can do is to make educated guesses.
The most likely reason why Americans became much more conservative in the 1970s is because the country had become so liberal in so many ways during the 1960s. In other words, the conservatism of the mid to late ‘70s was a backlash against the liberalism of the 1960s.
One aspect of the backlash was economic. During the 1960s, the government had instituted a number of very liberal programs. Perhaps the most notable of these was President Johnson’s “Great Society” program. This dedicated the government to improving society in important ways, including the eradication of poverty. This is when Medicare and Medicaid came into existence, for example. We can argue that these moves towards liberalism and “big government” provoked a backlash in the 1970s.
Another aspect of the backlash was cultural and political. The 1960s, of course, were a decade of massive cultural upheaval. There was the Civil Rights Movement. There was the “counterculture” and the hippies. There was the anti-war movement. There was the women’s lib movement and the sexual revolution. There was violence in black areas in the “long hot summers” of the mid to late ‘60s. All of these things contributed to a sense that traditional values were being lost. We can argue that these changes led to a backlash in favor of more traditional ways.
Thus, we can argue that the conservatism of the mid to late 1970s was a backlash against the liberalism and upheaval of the 1960s.