Because correlation it is. Compared to most other countries, Denmark is rich and benevolent, and the same goes for the rest of Scandinavia. As has been shown a number of times before, the more secular countries are, the richer they are, with some exceptions, like the US and Kuwait. → → →
Phil Zuckerman: In secular countries the core values of human right are being more successfully implemented than in religious countries.
Here's what Tom Rees concludes, blogging about his own paper, Is Personal Insecurity a Cause of Cross-National Differences in the Intensity of Religious Belief?
Nations have choices over how to look after the people at the bottom of the social pile. Those nations that choose to make this a priority, which inevitably involves shifting money and resource from the rich to the poor, lower the overall levels of stress. And when you remove the stress caused by their social situation, people tend to lose interest in religion.In other words, Tom suggests that removing stress makes a secular nation, and giving from the rich to the poor, you remove stress.
On a personal note, I can vouch for the stress that having no money brings, but not that it leads to religion. But then again, I have a whole internet chuck full of people helping remind me that religion is not the answer. If only they could also send money.