I think the internet and social media has had a lot to do with it. Twenty years ago I knew I couldn't be the only person on the planet who was not a believer, but until there was a modem in my home I had no way of connecting with others who shared my views. I live in a little rural pocket of the country where one can't swing a dead cat without hitting a church. Nobody around here talks about it if they don't believe.

Witnessing all the dumb shit that religions keep doing, from supporting the orange turd of a POTUS that we have to the covering up of pedophile priests and the catholic hypocrisy, and all the rich pastors and ministers, rejection of science, etcetera, who in their right mind would be a part of that crap?

Young people are more progressive and liberal. Religion is conservative dogma.

As people become more educated, they become less religious.

I think it's people seeing what harm fundamentalist religious dogma is doing to society, particularly when it gets into power (looking at YOU, Mike fucking Pence.) When you're brought up believing that your religion is one of peace, seeing that can be quite shocking. Not for all of them, obviously, but I think it's a contributing factor.

Influx of common sense? Lack of time/money to attend? Watching televangelists wail & scream on TV?

Churches aligning themselves with the GOP.

People are waking up from the bullsh$t.

Education, the internet, folks are more transient which puts them in different cultures within the same country, and a younger generation with some common sense?

Unaffiliated doesn't mean they gave up on religion. It just means they don't identify with one of the many churches.

As to why.... I'm sure the prosperity gospel preachers has something to do with it along with the popular pay-to-play preachers.

Education. Thusly it was a crime to teach a slave to read. And religion is the opioid of the masses (or currently the cell phone), & give a man a fish & he eats for a day. Teach one to fish & one eats for a lifetime (assuming global climate change doesn't kill all of the fish). So education results in the evolution in the ability to think. Thusly, "I think therefore I am an atheist".

I would like to think that people are just more educated and smarter than they were a generation ago. Maybe the internet is more than just dating sites and online gaming.

I think people are turning away from religion. Thirty years ago it was unheard of for people not to belong to a church.
It's a start.

How many can actually afford to to belong to a church any more?

Maybe people are starting to smarten-up...

Abuse of power by Churches & by organized religion, in general has destroyed faith in religion. Without fail it repeatedly proves that greed is he motivator behind the church. Instead of 3 letter word « God » it is the 3 letter word « Mob ». The church dies not have a “Flock”. It now has a “stock herd”. @ is using sex, trafficking, manipulation of “relationships“ & money. God is not some jackass on a pulpit, if there is a God, it would be greater than our ability ti define. The church has killed “god”. I don’t need someone to direct me to Be good, or decent, or to punish me for not working for them, which they always will do. Hence “Mob”. “Be a good female do this guy, then we’ll marry you to that one”, ad nauseum. Not joking here. Churches prostitute love. Whatever undefinable good or peace there is, if a person can define it completely, it is not “God”. That is limited. Knowledge, peace, beauty good, are boundless. The church tries to package & sell it. The church is a parasite. Non servium.

Social media is replacing the church as a community. The church is against gay rights and women controlling their bodies.

As man has learned more (not saying we are all critical thinkers), the old tomes don't fit in modern society as well as they did 100's of years ago. For example, the Bible is not available if almost 40 different versions. Churches have been fracturing more and more.

Education.

Social media, too busy with their noses to their phones. Instant gratification, number of likes, able to respond with impunity, etc. any or all and church just does not cut it. Non affiliated does not mean they do not believe in God, Jesus or the Bible just they are too wrapped up in their phones to give it much thought. The current administration is working very hard to insert the church into government and that is NOT a happy thing. Athena covered it for me. Bill Graham back when he was first making a name for himself was concerned government would co-opt religion for his own purposes - that thought seemed to have lasted a NY minute and he happy allowed it and babied it along.

The age of reason.

The internets ability to spread truth quickly.

Maybe it is because their not going to church!

My best guess is that more people are simply unaffiliated. Here in Colorado Springs, home of "Focus on the Family", numerous religious "colleges" and where Paula Poundstone did a bit mentioning being taken aback by finding 8 pages of churches in our local Yellow Pages: we are listed as having 66.6% "none" for religious adherents in city-data.com (pic).
On the same site I checked Albuquerque, NM (where I've been thinking of moving next year); 47% Hispanic population but 53% in that same "None" category for religious affiliation. Not to say all Hispanics are Catholic/religious, but from my experience it seems a sizable number of them prefer to "vaya con Dios". Maybe there is a prefer-not-to-answer option for the religious question, maybe people are leaving it blank. In either case it's pretty obvious there aren't as many atheists running around as this type of news would have us believe.