It's not just the United States in which Christianity is on the decline. The same thing is occurring in every economically wealthy 'First World' country.........Europe/Canada.
The decline in Christianity is simple, people aren't interested. They are more interested in what the world has to offer them in the way of sex/money/power/gratification/materialism.....etc, than what Christ has to offer them.
Keep in mind that, when Satan tempted Jesus that he tempted Him with all the things that the world had to offer, kingdoms of the earth if only Jesus would bow down and worship Satan. Jesus refused, but the 'well-off' people in today's world are not refusing. They are 'bowing down and worshiping' the things that the world has to offer. They are making a clear choice of the world over Christ.
But all is not lost. Keep in mind,, that when Jesus ascended to heaven, He only left a handful of disciples to carry on the faith and spread it throughout the world. He sent them as sheep in to a world filled with hungry wolves. And, this may be what He is asking each of us to do today; To Spread His Gospel in a world filled with people who don't want to hear it.
I do see hopeful signs in the world. Countries that used to be fanatically atheist, during my lifetime, are coming home to Christ. This includes Russia, China, and Cuba. Christianity is also growing rapidly, by leaps and bounds in Africa and it is growing in India.
All people haven't lost interest. I can't travel 5 miles from my residence without running across 5-10 Christian churches. And their parking lots are filled on Sundays.
I think I have a final question. What are we, each of us doing, to reevangalize the country, our neighbors, our friends, and our families who have dropped away from the faith? If the answer to that question is 'nothing', then we/you are part of the problem.