We got sufficiently ahead(economically and scientifically) and became lazy. There is no reason to advanced those things unless it gives us a significant advantage over the potential enemies. Now that we are rolling in the luxury we do not feel motivated to make our nation-dick any bigger.
It is like the story of the tortise and the hare, except since generally all countries have the same desire to gain an advantage over potential enemies and reach our state of luxury, the tortise is not only going to pass the hare, he is going to evolve into a goddamn pteradactyl, leave us in the dust, and we'll have forgotten by then how to evolve into something faster.