Economics

A country that is at peace with most countries make the big bucks. America became the number one country because it stayed away from most wars that was raging across Europe.
It is a changing scenario now. Iran's missiles are more of a financial threat to America than a military one. Iran now will have a lions share of the weapons market in the middle east and other islamic countries and this market was mostly American (and European) before. And really missiles sell better than nuclear weapons. Like I have said before nuclear weapons are a waste of money (perhaps serves as a deterrent) and only mad men will use it.