The launching of Sputnik 1 kicked off a real space race, as Americans were shocked by what the Russians had been able to achieve, especially with fears of possible nuclear missiles raining down from space. The cold war atmosphere of the time also made beating the Russians in all things a primary goal, with money being almost no object. Vast sums were thus poured into the US space program, ultimately culminating in the moon landings as well as the unmanned probes to the planets.
But the disappearance of the USSR removed this imperative. Now the main US imperative on the world stage is the defeat of muslim extremism, but this opponent is not competing in space at all. Other than that, the primary goal is saving US tax dollars, in order to cut taxes or at least avoid tax increases. Doing as much as you can for as little cost as possible is now the main focus. If projected costs of any missions are too great then those missions just won't happen. To send men to Mars or fund any other vastly costly space adventure is not really on the cards because no matter what the theoretical talk of it might be, the tax dollars will not be forthcoming. US leaders need to be voted in to power by a public that doesn't like paying unnecessary taxes.
What it will take is another undemocratic regime akin to the USSR, putting national prestige way ahead of any concerns about it's own people's tax rates, to finally pull off something spectacular and shock the American people with another Sputnik 1 moment. Thus far China looks like being the most likely candidate. For the time being it appears to still have some catching up to do. But if at some future point -after spending vast sums - it stuns the world by, for example, successfully landing men on Mars, it will likely create a new imperative in America, a new desire to pour money into space exploration to catch up and compete. Then perhaps we'll have a new space race.