The first reason is the absence of an established church in America. If all orthodox Protestant sects were to be free to worship, then others, such as Catholics, Quakers, Unitarians, and Jews could rightly claim similar rights. Despite residual social discrimination, non-Protestants were free to worship and had full political, civil, and property rights. Catholics, Jews, and later Mormons prospered in this country as had not been the case in Europe where Jews or Catholics were in the minority.
The second is the effects of the 18th Century Enlightenment in Britain and its colonies, in reaction to the religious wars and persecution of the prior centuries. Jews were permitted to return to England and settle in the American colonies, non-Anglican dissenters were freed of restrictions on worship and business activities, and slowly but surely, Catholic emancipation was effected.
The third reason, since the late 19th Century, was the rise of dispensationalism in conservative Protestantism. This theological system clearly distinguished the role of the church from that of the Jewish nation. It holds to the position that national Israel remained in covenant with God. Prior to its rise, conservative Protestants largely followed covenantal, or replacement theology, which held that the church replaced national Israel as in covenant with God. Under covenantal theology, the Jewish nation were no longer in covenant, and were no different than any other nation or ethnic group.
Excellent summation!