In Vol. I of The History of the Decline and Fall of the Roman Empire, Edward Gibbon explains why Christianity spread and took over Rome, while Jews remained a tiny minority in the empire. Here's one of my favorite passages from Gibbon's discussion of Judaism:

The Jewish religion was admirably fitted for defense, but it was never designed for conquest. … The obligation of preaching to the Gentiles the faith of Moses had never been inculcated as a precept of the law, nor were the Jews inclined to impose it upon themselves as a voluntary duty. In the admission of new citizens, that unsocial people was actuated by the selfish vanity of the Greeks, rather than by the generous policy of Rome. The descendants of Abraham were flattered by the opinion, that they alone were the heirs of the covenant, and they were apprehensive of diminishing the value of their inheritance, by sharing it too easily with the strangers of the earth.