Why is Christianity on the decline in the USA?

People are starting to see it for the silly cult that it is.  And its followers prove it.  When the cult has nothing of value to offer people, they always have to fall back onto empty cult threats of hellfire and eternal suffering with the devil.  The cult is as empty as the threats are!

cause a lot of people dont believe in it

"But mark this: There will be terrible times in the last days. 2 People will be lovers of themselves, lovers of money, boastful, proud, abusive, disobedient to their parents, ungrateful, unholy, 3 without love, unforgiving, slanderous, without self-control, brutal, not lovers of the good, 4 treacherous, rash, conceited, lovers of pleasure rather than lovers of God— 5 having a form of godliness but denying its power.(2 timothy 3:1-5)

Better education and growing intelligence greatly helped by the terrible intolerance, hatred, bigotry, homophobia and persecution by Christians towards others! No DECENT persons wants to spend a minute in such company let alone eternity!   

Matthew 24:12-13 
And because iniquity shall abound, the love of many shall wax cold. [13] But he that shall endure unto the end, the same shall be saved.

Education.  Religion is not supported by fact,

Because of the great falling away from the faith.

2Thes.2
[1] Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him,
[2] That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand.
[3] Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
[4] Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
[5] Remember ye not, that, when I was yet with you, I told you these things?               2 Thess

Facts not in evidence  fallacy so the question is moot.

We are living in the end days. https://www.youtube.com/watch?v=LfTHLemTVWU

because Jesus even said that there would be few who are saved or walk the narrow road that leads to life but that many will take the wide gate that leads to destruction Matthew 7:13,14. Thanks for the question.