This BBC article shows that Islam is the fastest growing religion in Europe due to immigration and above average birth rates.
Look at the percentages and remember that Christians who have a personal relationship with Jesus and are willing to tell their friends about Him are averaging at only 2.4% of the population throughout Europe with over 19 countries averaging less than 1%.
Also, Europe is very influential and gives a great advantage to connect with the Muslim world. As these great multitudes immigrate to Europe, they are in a more religiously free environment and are usually more open to other ideas and thoughts. These new thoughts and ideas can be taken back by them to their families who are still in restricted countries where we could not connect with them as easily as Europe.