It seems that when Jews were scattered away from Palestine, for a while they were readily adapting local languages as their vernaculars. In Spain it was Spanish-based Ladino, in Germany Germanic Yiddish, in Persia Fors (Farsi, later to be called Bukhari)). Later on, the same Jews would move to other countries, but now they would shlep their newly created languages along with them. Thus, in Poland and Russia they would keep Yiddish for many centuries, in Bulgaria or Greece, Ladino, in Uzbekistan, Fors.
Any thought as to why such a (suddenly acquired) conservatism?