The post-WWII world order fell apart when the Berlin wall fell.
That order was defined by the tension between the east and the west during the cold war, and it can't be overstated what an existential threat communism was perceived as by the west during this period.
Half a century of American politics were based on the premise that if the US did not win against the Soviets, the entire world would live under a world wide oppressive communist regime. Freedom would be a historical footnote.
This meant that the US urgently needed to have its shit together at all points, not just function well, but also look good. The war against communism was fought in the hearts and minds of the world's population.
Since the Soviet union fell, and with nobody to keep the US honest anymore, it's arguably let itself go.
In part it's metaphorically stopped sucking in it's stomach, to hide its flaws and present itself as flawlessly glamorous. Some of the American decline is problems that have existed for a long time that have now been brought to the surface; but I think the lack of competition is also just as big of a factor.
New powers have emerged in the vacuum left behind the Soviet union, yes, but these are all knock-on effects, and I don't think they're perceived as the sort of existential threat as the communists were. Some are trying to paint them that way, but it's not nearly the same.