Because this country was founded on God and Christian beliefs.
Out of all the people in the world who wouldn't mind immigrating to the USA --- why do we close the door to some and allow these who have no intention of assimilating with Western Judeo-Christian culture and values in? If we need a different type of religion --- let Hindus, Buddhists, or animists in --- some kind that can co-exist with Christians and Jews. We don't need to import all the problems of the Middle East.