Because civilized, formerly Christian countries are getting soft and weak, and losing their faith. The savages see an opportunity to rush in and force everyone to accept their subhuman way of life.
If this question has to asked, then we are in far more trouble than I thought. Bin Laden may have failed initially with his grandiose Muslim manifesto, but his vision is back on track and not letting up. Our enemies always use time and opportunity to their advantage. We never learn and that very well be our ultimate demise.