The easy explanation is to say atheists are smarter. However, this ignores the societal implications of atheism and religiousness.

I think atheism is more the philosophy of the self sufficient than the non self sufficient. So, people who would be atheists if they had the means end up not being atheists when they do not (at least in the general case). However, the converse is not true of being religious. Therefore, the religious label can encompass a wider range of people.