There are several good points here. I tend to think it is rooted in the fact that so many original colonist were religious extremists of their day and the belief that this country was a gift from their god--and that their god plays an active role in its success (or failure); the "City Upon the Hill" and "Manifest Destiny" line of thinking. I was raised to think that our founders were dedicated, Protestant Christian men of god who were doing god's work when they founded this nation which was based on "Judeo/Christian values;" and, furthermore, is part of this god's plan to spread the gospel of Jesus to the ends of the earth. Imagine my surprise when I learned otherwise.

This a good question, and I've wondered about it too.
Why is the majority of the US still clutching at this silly ancient belief system?
Why do our politicians feel the constant need to proclaim and reinforce their faith?
Why do we rarely, if ever, hear of an atheist running for office?
Why is religious propaganda allowed to be plastered all over our public buildings and other properties, even though it's specifically against The Constitution?

There is still a 'mob rules' sort of mentality here when it comes to religion, especially in the Bible Belt.
Many non-believers are downright afraid to come out for fear ostracization from friends and family, or even the entire school or town.
With everything being as lopsided as it is toward Christianity, and the fact that new younger people are afraid to come out, it will remain this way for some time.
If more atheists would stand up and be counted, we could get more secular people running for office and winning, and that could create a more secular government.

all the way back to the founding of the American colonies Most of them were founded as religious communities' and most of those religious colonies were off-shoots of evangelical Calvinism. Religion also played a major role in the America Revolution, as disclosed in Alan Heimert's book, THE AMERICAN MIND FROM THE GREAT AWAKENING TO THE AMERICAN REVOLUTION.

We use the same school system as Nazi Germany which was also a ridiculously religious country. The Prussian education system brainwashes the students of what to think making religion easier to swallow.

The wealthy class, perhaps, use religion to keep the masses under control by giving them a God to whom they can go to release their frustrations that might otherwise be directed at the wealthy class.

I saw a great documentary by Niall Ferguson. His thesis is that it's all about the competition. Most countries have state religions, so you have a choice to either practice the state religion or practice nothing. The United States allowed religious freedom from day 1, so churches were allowed to innovate and morph their bullshit stories to gather in the most people to their church.