Why is Atheism and Agnosticism so much more prevalent in Europe than in the Americas?

Why does Christianity have so much more influence in the former European colonies (USA, South America, etc.) than in Europe itself? Compare for example the UK, where 35% of the population do not believe in God to the US, where only 6% of the population do not believe in God or a higher power. Similarly, 18% of the people in Spain are atheist compared to less than 2% in countries like Paraguay, Colombia and Peru. The European countries with the least amount of theists are the Scandinavian countries, the Baltic States, the NL, Czech Rep., France, Slovenia and the UK. In all of these countries, less than 40% claim to believe in God.

Why do you think Europe developed in such a different way than North & South America? It seems that in the Americas, Christianity has only become more evangelical and fundamentalist in the last decades whereas in Europe, the influence of religion on society has greatly declined.

Sources: http://www.gallup.com/poll/109108/belief-god-far-lower-western-us.aspx, http://en.wikipedia.org/wiki/Demographics_of_atheism#cite_note-35