Hey, I wrote an answer to a question along the same lines (about the decline of religious people in the Arabic world), the answer I provided there is still valid for your question even if it has some precisions toward that region. However, the thought behind it remains the same :
The more your primary needs are fulfilled (see Maslow's pyramid of need's hierarchy), the more you are likely to turn away from Religion.

I don't think it's a matter of pure Historic fact (a result of Geo-political moves) but more a result of social evolution (but can be linked to History ; anthropology). Thus I don't think my answer is the answer, but more a way to see things or maybe part of the answer.