I'm formerly Muslim (new Christian) but I was born and raised in the U.S., and even I noticed a decline over all.
I'm not sure just one thing is causal, but rather probably many factors.
I think the more human beings learn, the more we can do through knowledge and technology, the more people become their own god.
I think Babel is a good similtude here.
Working together the people under God's curse, in rebellious pride, wanted to make a name for themselves to avoid being scattered over the face of the whole earth and decided to do so via a monument that could reach the heavens. (Genesis 11:4).
God punished them by giving them exactly what they didn't want - He scattered them over the face of the whole earth, a lesson in humility for them, a cut to their pride.
Anytime man becomes over confident and over prideful, God allows it for a time and then allows their works to come to nothing in order to remind them they are indeed not God, and do in fact have need of Him and His path....
It is perhaps this is where we are again. Might be a time where God lets out the reigns for a while, with a moral lesson coming at the end..
So I guess my answer is this, it's the pride of man causing the decline atm.