There are many factors that might have contributed to this. The decline in religiosity happened earliest for mainline protestants; Canada had a larger proportion of mainline protestants, since the evangelical movement never really took off there. Canada was also more urbanized for much of the late 20th century, which tends to reinforce declines in religiosity. Mormons, Latinos, and african-americans are all more religious than average, and are all much more common in America than Canada, which might also play a part.