What about the factor of disillusionment with the rude discovery that there exists a great disparity between the practices of "popular Buddhism" so widespread these days and that of what the Buddha did actually teach as recorded in the Suttas (Nikayas)? 

For those who are strongly faith-based, they opted for simple straightforward modern theistic religions. For those who are reason-based, they prefer to be free-thinkers. Thus the increase in both Christianity and free-thinkers.