The USA rose to Superpower status because it was an incubation nation for ideals and innovations, and a Government that cultivated an environment that allowed them a place to truly take off more so than other nations from the 1800s through mid-1900s. It wasn't because there was no one else to compete with, it was just that every other nation was so ridged over millennia of culture that they just couldn't compete with a more fluid government and set of ideals.
When you combine the above, it's no wonder that the nation as a whole will become more prosperous quicker than surrounding nations. This was true with Rome, China, France, and Britain in the past when they held the great thinkers and flexible governments at the time that allowed innovations to flourish.
Like them, corruption and complacency win out and allow other nations take to their place.