This may seem like a "Captain Obvious" reply, but, a lot of this has to do with economics. As the cost of living goes up (not to mention flat wages) it is difficult to provide for more than one child. The middle east, on the other hand, is still (in most ways) economically backwards so it doesn't matter as much. Muslims can immigrate to another country and get government assistance to support their children, making it easier for them to have a large family. The prison system also helps spread Islam in the United States.
I'm not at all worried about these demographics because Christianity will most likely always stay dominant in the Western Hemisphere. The Latino population in the US is dramatically climbing, making Christianity more wide spread.