Any healthy, growing society will look outwards, attempting to expand it's envelope. As far as the United States is concerned, that was out attitude for the first 200 years or so, culminating with the space program, and the successful moon landings in the late 60's. In 1969, it was quite reasonable to assume we would have men on Mars & beyond by the year 2000.
Our literature of the time reflected this, there was a great deal of very good science fiction written in the U.S in the 1st half of the 20th Century. As our society looked outward, so did our entertainment.
However, in the late 60's and 70's, the U.S. began to decline. By the 1980's, we had abandoned our exploration efforts, preferring instead to waste our resources by throwing them down endless money pits of social service programs, rather than investing in our future. As this decline has accelerated, we stopped looking outward in our entertainment as well. Most of what sci fi has been created no longer deals with what we'll find when we start moving out into space, but instead focuses on how life will be after the coming destruction of our society.
The US has been in decline for 50 years now, and we're near the end. Just as with a living person, a society at the end of its life stops looking outward and turns inward.
