Why is organized religion declining in the U.S.?

Do you only show up at church for Christmas and Easter?

A Gallup survey shows that between the late 1930s and late 90s the percentage of Americans who belonged to a house of worship was at around 70 percent. But in the last 20 years, that percentage dropped to 50 percent. 

Millennials make up the biggest percentage of those who don't identify with a specific religion. That drop doesn't necessarily mean Americans are not religious or believe in a higher power. More than three quarters of Americans, 77 percent, still identify with some organized religion. 

However, there is a growing number of young people who are not following their grandparents' and parents' footsteps. They simply in some cases don't believe in organized religion, but the question is why?