Bob, this is pretty much what happened to the space program. The cold war ended, and america's biggest, baddest enemy, Russia (USSR if you want to get historically accurate) just either ran out of resources or focused on other things besides competing with the states for space.
So where does that leave America, where (or more accurately, when) "men were men, women were women" and the country had something it wanted to hit first? Honestly, the states have never cared more about space travel then when their ideological arch nemesis got curious about what might be up there. Russia see's whats up, seems satisfied, and just sits back to let America piss all over them, in a figurative sense, with the moon landing and mars missions and whatever else.
In short though, I think its summed up by this. They don't have anyone to 1-up now. There is no big contest for whoever has the bigger dick, like in the cold war. Now, there is simply no one to show off to, no world that needs to impressed. Sure they explored for a few years after that, then came other presidents who were more content to do other things, such as nothing. Now they have no purpose to get into space, if no one wants to beat them to it. And by no one, I mean Russia, and with an american president that doesn't want to be outdone in a pissing contest for mars.