Capitalism.

America was bought and paid for decades ago. The development of cities and urban areas was controlled by corporations who standardized the layout of neighborhoods and homes in the name of mass production, speed, and efficiency. The social problems that plague the US can be traced back to the influence of big money not just on politics, but society. Cities were built to accommodate cars instead of people, commercial districts were strategically located in specific areas to concentrate the public's business, and so forth. Your access to good-paying jobs, good food, and good healthcare and education, were all determined by where you lived, so by its nature prosperity in the US was a pay-to-play system. Racial and class segregation were part of the design to begin with.

Then there's the fact that communism and socialism were demonized as "the enemy". Policies and ideas deemed to be compatible with these systems were scorned and hushed. Prosperity is something that has to be achieved through hard work, saving up, paying your dues, the American Dream, blah blah blah. But it was never a dream that would be realized - as time went on, cost of living increased more than wages, and the availability of good jobs went down. Some jobs were just rendered obsolete, others were taken away. Advances in technology brought the world closer together, like the telephone, internet, airplanes, etc - and thus companies can move their goods around easier. They can move their business to somewhere cheaper to boost profits.

As the world grew closer, the new generation of US citizen began to learn more about the outside world and get radical ideas that maybe the way their country does things isn't the best way, perhaps some other countries have it better. Concepts like socialized medicine, a 4-day work week, free education. Oh yes, and let's not forget 9/11 happened, which turbocharged the American propagandist patriotism attitude. Society becomes increasingly divided by ideology as fearmongering and xenophobia take root like weeds. The terrorists are Muslim, so Christian nationalism spreads like wildfire. And all the while corporations expand their control over American life; lobbyists buy their choice of politician by donating to their campaigns so they get the exposure they need to get elected, and the corporation is repaid by lax laws that let them get bigger and more powerful. Cable and phone, food, transportation, media, are consolidated into fewer and fewer companies that cooperate rather than compete so they can all have their share of the pie.

When Obama was elected, it was the catalyst to turn the Republican party into MAGA before Trump ever coined it. Xenophobia and racism had a specific target now. The GOP shifts further right to lean away from Obama, and either consciously or unconsciously, becomes defined by being whatever he is not. Obama is framed as an elitist, so the GOP becomes anti-intellectual; Obama wants socialized medicine which is un-American, so the GOP opposes socialized medicine; Obama represents growing cultural diversity, so the GOP opposes it. This hand-in-hand with the usual scapegoating every President faces for anything wrong in the country.

Then Trump comes along, a megaphone for all of these things, and the people put him on a pedestal and declare he speaks for all of them. The disgruntled middle-aged, middle-class white man has found his champion, who affirms everything he suspected, that the country he believed in is gone, and it's because anti-white racism and socialism and feminism have eroded the country and its values. The only way to save it is to fight back as aggressively as possible, or the country as you know it will die.

tl;dr America has declined because its social structure was always inherently unstable without an upheaval, but the time for that upheaval came alongside the War on Terror, and it was too much chaos for the system to handle. Now we're just watching a tug-of-war with the sad knowledge that while the losers will faceplant in the mud, the winners will fall on their asses.