Perhaps the number of Jews remained small because they did not feel the need to impose their religion on others on a mass scale. As for the benefits of assimilation which you view as a great favor to Jews, a "key cultural condition" in Jewish history was strongly to discourage intermarriage (10,000 Year Explosion, Cochran and Harpending, 2009, p. 193). Without benefit of your big favor, "Ashkenazic Jews have the highest IQ of any ethnic group known" (ibid., p. 188). "Although they [Jews] represent less than 3 percent of the U.S. population, they won 27 percent of the U.S. Nobel Prizes" over the past two generations and 25 percent of A.M. Turing Awards (ibid., p. 190).
Italian FM arrives in Iran to promote economic, political ties (Reuters)
from the article: 'Lost Jews' video shows Israeli chutzpah towards Diaspora Jewry