The reason is, it was founded on religious the principles of liberty, and the bases for that were the Five Books of Moses, the Prophets, the Psalmists, as well as the New Testament. Today, America remains the most religious country in the West as compared to Europe. But with the erosion and decline of religion in America, the result of the abandonment of religious practices, rituals, and traditions, in favor of the European model of universalism, anti-Semitism is insinuating itself into our culture from both the right and the left. The extreme-right wants to destroy Jews, while the extreme-left wants to destroy Judaism.
David Nirenberg, Dean of the University of Chicago Divinity School, made the point that even in cultures where there are few or no Jews, Jews are still hated. Case in point, in many countries where Islam is the official state religion, anti-Semitism is nurtured in the home, taught in the school, preached in the mosque, and legislated as official government policy. Therefore, countries with a smattering of Jews or none at all, anti-Semitism cannot be directed at individuals, rather it is the idea of Judaism itself that is the target of their derision and disdain. Nirenberg calls anti-Semitism in those cases, not anti-Semitism, but rather anti-Judaism.
The Founding Fathers of the United States were people well versed in the Scriptures and understood that Jews, Judaism and Israel are inextricably bound by history, by covenant, and by indigenousness to the Land of Israel. To deny that reality is to disconnect the body from the soul of the Jewish people. That understanding found its way into some of the United States’ sacred documents and that is why America today is still the least anti-Semitic country in the West.