That, as a matter of fact, has very little to do with exposure to other religions but a lot with embracing their own culture and religion in an environment that is both alien and sometimes even hostile.
Same happens to Ex-Yugoslavs when they emigrate to Switzerland, as well as probably Latinos when they emigrate to the US. Second and even third generations of immigrants are often much more conservative when it comes to their culture and religion than their parents or grandparents who originally emigrated.
I don't think that natural resources is a helpful idea here. Western Europe is more or less devoid of any natural resouces (no need to take into account the little bit of coal Germany and France produce, because it isn't even chickenshit compared to their economic output), yet it is here were we find the lowest rates of theists/religious people.
Per capita income and especially it's distribution is most likely a better marker. But ultimately I believe that the downfall of religions in western Europe is a result of hedonism. I wouldn't even say that people believe less, but they believe less in a certain religion. Instead, they pick whatever damn well they please, be it a little bit buddhism there, a little bit cabalism there and of course tons of esoteric hocus pocus, all that based on christian believes they once learned about in school.
At least that's the impression I get when I look around at my friends. Most of them do believe in something, but don't really have a clue in what they believe.