America is basically the only developed country (as opposed to countries like Britain, France, Japan, Australia, etc.) that is still "heavily religious" as well as the only developed country that still tends to have problems with Evolution.

What theory or theories do you have to account for why America seems to be the one exception?

I think it's because America has such a large population and because the geography, environment and climate allow for many of these people to live in the "middle" or rural parts of America and within smaller communities it seems to be more natural or easier for religion to not just continue to exist but flourish.

Also I think because, unlike other countries, America has more of an 'insular' feel whereby its own cultural ideals and traditions are passed down and circulated while cultural ideologies and customs from other countries doesn't tend to integrate so much within U.S. culture as what they do within other countries.

Thoughts?

I think the main reason is due to the original colonists. Many of them were people who were very devout in their unusual religious views, and were escaping the religious oppression of Europe. This view of having to defend your religious views strongly has carried on into today's American culture. That is my theory.

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... I am surprised at your insolence in writing to me at all. You know, as I know, that I bought this constituency... may God's curse light upon you and may it make your women as open and as free to the excise officers as your wives and daughters have always been to me while I have represented your scoundrel corporation.
I have the honour to be... your obliged humble servant, Anthony Henley
- MPs reply to constituent, mid 1700s

Alexis De Tocqueville would say it’s because we have separation of church and state. It is this separation and the non-interference of government in religion and religion in government that has enabled religion to flourish in America.

Well, that is true compared to Britain, Japan, etc, but France is a secular country, as are quite a few other European countries.

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... I am surprised at your insolence in writing to me at all. You know, as I know, that I bought this constituency... may God's curse light upon you and may it make your women as open and as free to the excise officers as your wives and daughters have always been to me while I have represented your scoundrel corporation.
I have the honour to be... your obliged humble servant, Anthony Henley
- MPs reply to constituent, mid 1700s