There are many possible reasons that could be cited for this development. Among them are:
- Environmental conditions had gotten progressively worse. For example, the Cuyahoga River in Cleveland, Ohio actually caught fire in 1969. The idea that a river could be so polluted as to catch fire was the sort of thing to make people take notice and become more interested in environmentalism.
- People were getting richer. When people have more money, they can afford to be more interested in causes. By the '70s, Americans were quite affluent and had the wherewithal to pay attention to things beyond their pocket books.
- The influence of the '60s. During the '60s, there were many reform movements going on in the US. People were trying to improve US society in various ways. This spirit of change lived on into the '70s and, as the Vietnam War and the Civil Rights Movement waned, things like environmentalism took over.