America in, say, 1910 was what we would call a "developing country" today. It didn't even have a highway system. Electrification was still woefully incomplete, many people lived in rural poverty or in tenements, even sewer and water systems were incomplete, and tens of millions of Americans didn't even have indoor plumbing. Something that's worth pointing out that is not well remembered is that municipal water systems typically pre-date water treatment. And while it was definitely superior to have water pumped in from a hopefully clean remote location, for many years it was possible for water born illnesses to be spread by the water pumped into people's homes.

The very first instance of municipal water being disinfected with chemical treatments was in 1908. Decade by decade the US passed legal standards for municipal water and cities began to implement them, but it took until the mid 20th century for it to be ubiquitous. Indeed, consider the polio epidemic of the early to mid 20th century in the US. Part of the problem was that advances of public sanitation were common but not yet bulletproof. Polio became less endemic such that children were no longer exposed to it in their youth, acquiring active immunity against it when they still had the benefit of maternal antibodies passed through breast milk, for example. The result was that an endemic disease transformed into one of periodic epidemics, with more severe cases. Because many drinking water sources were untreated and could be sources of infection, and because most pools were simply filled with untreated water. It took decades for public sanitation to reach levels where most people weren't drinking or interacting with untreated water constantly.

Through the early decades of the 20th century you saw huge advances in the "development" of the US, and especially in public health and quality of life. In 1950 the "typical" American household had clean drinking water, hot and cold running water, toilets connected to a sewer system, a refrigerator and freezer, an electric or gas range, an oven controlled by a thermostat, a radio and television set, a record player, a car or truck, indoor electric lighting. On top of that they had access to higher quality fresh and frozen food. Something as simple as being able to eat "fresh" peas that have been heated up from being flash frozen (instead of having to eat canned peas) is a major quality of life change, and you can apply that times a million little instances throughout early 20th century life. At the same time major changes like jet planes and widely available air travel, supersonic aircraft, space exploration, and new materials like plastics promised a whole new landscape of advances and improvements for the second half of the 20th century.

Those advancements transformed the life of the average American, raising tens of millions of people to a quality of life that was only achievable by the rich in the late 19th century. It was these changes which people saw as carrying forward into the near-future, creating a world that was even yet cleaner, more filled with common-place luxuries, less disease, more medical miracles, etc, etc.

On top of that there were many major changes made in America post-WWII, such as the GI bill, creation of sub-urbs (levittowns) which dramatically affected the lives of millions of Americans from the late 1940s through the 1960s. Many of the beneficiaries of those policies (read: recipients of government handouts) saw them as natural and normal examples of "progress" and as natural and normal extensions of the benefits of industrial development and mass production that had occurred through the early 20th century. They saw corporations and the government as working hand in hand to raise the American standard of living and make the world a better place. Ford and Chrysler built the tanks that won the war against the Nazis, and they also built the pretty car sitting in your driveway. It was easy to see America, the American government, and American industry as all working together to help make your life and everyone's lives better through industry and advancing technology.

That's a lot of what drove the obsession with the promise of a technologically and industrially more advanced near future from the perspective of the 1950s and '60s, especially among those that had benefited so greatly from government handouts and favoritism in the preceding decades. This obsession occurred with either the intentional or passive blindness towards a lot of other things that were happening at the time, such as widespread segregation and racism in the US, abuse of government and corporate power, US government support for fascists and despots overseas, the rising problem of industrial and agricultural pollution, and on and on and on. Ultimately the contradictions, blindnesses, and limited inclusiveness of that "world of tomorrow" mindset is what lead to its unraveling. The Vietnam War showed that the US was neither omnipotent nor unquestionably a force for good. The Civil Rights Movement showed that even a nation capable of landing humans on the Moon and flying aircraft at Mach 6+ didn't somehow make fundamental problems of injustice and bigotry evaporate. The advance of technology, industry, and even of affluence didn't magically solve every problem in America, and they didn't take away or replace all of the hard, complicated, necessary work it would take to fix those problems.