The precise reason for this may not be known. I think it has to do with the emphasis Western Europe and the United States has placed upon "Family Planning," "Birth Control" and the availability of abortion.
 

Islam is opposed to abortion:

"Say: Come, I will rehearse what God has really prohibited you from: Join nothing as equal with Him; be good to your parents, kill not your children on a plea of poverty; We provide sustenance for you and for them; approach not shameful deeds, whether open or secret; take not life, which God has made sacred, except by a way of justice and law ~Koran (Chapter 6, Verse 151).

"Kill not your children for fear of want; it is We who provide sustenance for them as well as for you; for verily killing them is a great sin. ~Koran (Chapter 17, Verse 31).

In a larger sense the Christian population in Europe is experiencing a negative growth rate. Here are some population projections taken from wikipedia.

According to Don Melvin

Europe's Muslim population (without Russia) has nearly tripled over the last 30 years, to about 23 million, and experts predict it will double again by 2020. In 2005, the EU-25 had an overall net gain from international migration of +1.8 million people. This accounts for almost 85% of Europe's total population growth in 2005.

According to Omer Taspinar

Today, the Muslim birth rate in Europe is three times higher than the non-Muslim one. If current trends continue, the Muslim population of Europe will nearly double by 2015, while the non-Muslim population will shrink by 3.5 percent.

According to Esther Pan: By 2050, 1 in 5 Europeans will likely be Muslim.