It is called history.
Reasons for these Jewish migrations:
- Persecution and expulsion in Christian Europe: Jews in medieval Europe faced various forms of persecution, including accusations of blood libel, forced conversions, and outright expulsions from countries like England, France, and Spain.
- Relative tolerance in certain Muslim lands: While treatment of Jews varied across different periods and locations under Islamic rule, there were times and places, like certain periods in the Ottoman Empire, where conditions were comparatively more tolerant than in Christian Europe. Jews under Islamic rule were often granted "dhimmi" status, which offered certain rights and protections, though it also involved specific obligations and restrictions.
- Opportunity and prosperity: Jews moving to Muslim lands sometimes found more opportunities to engage in various professions and thrive economically and culturally.
True. There were Jewish communities living under Islam since Islam began, There were Jewish communities living under Christians since Christianity began but they suffered restrictions and sometimes expulsion. You can't be a devout Jewish man without being able to read the Torah so literacy was commonplace among Jews.