This is a question that many people have tried to answer, and now it's my turn to expound a little on the subject. After all, for good and evil, not only the U.S. is the only superpower today, but are also the most influential country in the world, and what happens there inevitably influences the rest of the world in one way or another. Moreover, much of the culture that "consume" (film, music, literature, etc..) Comes from there.
For those who do not know, the United States - by far - the country of "first world" more religious world, in general religiosity of a country is inversely proportional to the development, wealth and access to education in it. The United States is the exception, as this chart ( source ) shows:
Notice that the U.S. position "is a clash with" there, is that religion would expect in a country much poorer and less developed. Why is it so?
There are two general explanations suggested, and I think the reality is a combination of both. The first is that, unlike most other countries of "first world" (Europe, Japan, etc..), The U.S. has never had an official religion. The "founding fathers" were careful to prevent precisely this - the famous "separation of church and state" - and that's why the U.S. is not "a Christian country", at least in official terms, despite a proportion of practicing Christians much larger than, for example, Portugal - officially a Christian country (Catholic variant), but that the overwhelming majority of people are non-practicing.
This seems contradictory, does not it? A country is not to have more religious (or never had) religion? Would not expect otherwise?
The point is that a religion causes complacency, causes comfort, makes religion a tradition that most people have, rather than something to be taken really seriously. In short, creates non-practicing, just like we have in Portugal, or as is the case in England. It is assumed that everyone belongs to X religion (Catholic, Anglican, etc..) Birth (or baptism), and do not think much about it; their own baptisms and marriages in the church, something that is more is by tradition than anything else (I remember going to a christening in the family that the church being full, there was probably no Christian here below ... 60).
Instead, in a country like the U.S., the various religions have, and had from the beginning to compete with them over the decades perfecting their methods of winning adherents and awakening passion - in many cases, I would say fanaticism - them. It is a kind of Darwinian evolution (ironically in large part on who rejects the reality of it): the religions that have survived until today in a competitive environment so are very efficient to raise members and make them take it seriously.
The second question is this: although the U.S. is a country "rich" Americans are a people who, in their overwhelming majority, live in fear. I will not enter here into political opinions, but because of his conservatism and the fact of being a country much more "right" than most of Europe, Americans are by nature averse to social security systems (I refer here the concept, not the Social Security (in capitals) as a service or organization), a person can more easily be fired without "just cause" or reasonable compensation payments (except in the case of executives, of course), and any time a person can lose everything because of a disease. And, note, I'm talking about middle-class families with home and car, I'm not referring to real poverty.
Not to mention the truly "rich", the majority of Americans live with knowledge that can lose everything at any time and for reasons completely beyond their control. It is natural, always living with a certain level of fear, uncertainty and stress, people seek security in some way ... and the idea of a deity who cares with us, who "has a plan for us," which is always there for us even if everything else goes wrong, it is very tempting ... and very comforting.
It said the villain in "The Name of the Rose" by Umberto Eco: fearless, people do not need God (hence the whole "plot" to cover up the existence of a book of Aristotle on comedy, since nothing so void and fear as laughter). Maybe that's why that the most religious in the U.S. - basically, the Republican party - have always opposed to any kind of social security - Social Security itself in 1935 (introduced by Franklin D. Roosevelt), the Medicare system in 1965 (Lyndon B. Johnson), and now the renewal of the health system (Barack Obama, all Democrats). No doubt much of the opposition is political, in order to harm the Democratic government in order to be blamed for the problems this country and the Republicans win votes, but one can also argue that a society in which there is much more security and stability, where people know that if they fall there is a "network" to hold them, they never completely go down and they are not all the days of his life at risk of losing everything they worked for a lifetime, is a society that "need" much less "God", and therefore will tend to become less religious over time.
Incidentally, this is what I hope happens. 
Hence have been pleased by the success - until now - the reform of the health system in the U.S., it seems even go forward.
And if this idea - that I'm not the first to suggest - is correct, then the graph above can be interpreted in another way in which the United Estates are no longer an exception "strange" to the rule. The inverse ratio is between religiosity and wealth / development, but between religiosity and security that you have towards life. Means more uncertainty implies fear religion. So hopefully more safety implies less religion in the future.
