Why were the Jews singled out for persecution so frequently, exploring 1) the historical explanations, such as religous rivairy, envy of Jewish economic success, and making the Jews a scapegoat in times of political and social unrest or 2) the explanations of our sages as the weakness of Jewish religious life before periods of persecution and the providential guidance of the jewish people in exile?
One reason historically given for persecution of Jews with larger religious and ethnic reasons is the financial enterprises and successes that Jewish members of societies seem to enjoy. Shylock in Shakepeare's The Merchant of Venice is a illustrative example of this.
The irony of this branch of persecution is that Jews were in the various cities and countries as exiles; many cities and countries refused to grant Jews shelter from early centuries through to the years of World War II. While exiles in historic times, there were many law as enacted against Jews restricting the areas they could live in and the occupations they could practice. One occupation that was uniformly open to Jews was that of moneylender, an occupation no Christian wanted to undertake because the Scriptures warn against lending and borrowing money.