Secularism, Agnosticism. Priest’s and Bishops have had an effect as well.
There were more practicing people 100 years ago in the USof A and Europe than today.
The Church grew almost 7000 % in Africa and the South Pacific in the same period of time.
Its just my observation but I seem to think as people in Western societies become more wealthy, or disposable income, they've grown away from God. My church has just come back from Africa and the people the visited lived in literally mud huts but their faith in God was strong.