The short answer is that it became impossible to ignore. Decades of largely unrestricted pesticide use (DDT in particular), industrial pollution, fish kills in rivers and streams and lakes, contaminated groundwater and "cancer clusters" near industrial sites made the news repeatedly and continually. This sensitized and convinced the public that environmental protection was necessary.
This was true even in the early 1970s, as most people don't remember that Richard Nixon signed the Clean Air and Water Act, perhaps the most important environmental legislation to date. It had an almost immediate positive effect on air and water quality.
There were events that happened prior to the 1970's that assisted in the development of environmental awareness. Several factors occurred, such as the testing of hydrogen bombs and oil spills helped to bring attention to the importance of keeping our environment clean.
As people became more aware, Earth day demonstrations started popping up and this became a huge, momentous event across America.
There was also an energy crisis that was developing during this decade which prompted environmentalism. In 1973 there was the international oil embargo which caused a lot of panic because the United State's oil supply from the middle east was cut off. This was called the "Mideast Oil Crisis."
There can be many explanations behind why Americans took more interest in the environment in the 1970s. Certainly, one reason could be the residual effect of the counterculture of the 1960s. Environmental awareness or ecological ethic was an extremely potent aspect of the movement. While world peace might not have been fully able to have been realized with the escalation of the Vietnam War, environmental awareness was a part of the movement that could carry over and was portable from one decade to another. The government establishment of the Environmental Protection Agency was another social and political development which helped Americans to take an increased interest in the environment. Inspired by the work of Rachel Carson's "Silent Spring," the EPA was launched to ensure that the government would be able to take more of an interest in the matters of the environment.