One of the explanations makes perfect sense:
Church goers are increasingly being associated with conservative bigots. I started noticing it in the 1980's. This is a reason why many adults I know don't attend church anymore and don't want to be associated with any religion. Those of us who actually grew up with 'Christian values' wonder what happened to them and why hating gays and banning abortion are the only issues of the day.
"...they grew up in a period in which being religious meant being politically conservative, especially on social issues."
Putnam says that in the past two decades, many young people began to view organized religion as a source of "intolerance and rigidity and doctrinaire political views," and therefore stopped going to church.