One historical reason is that, hundreds of years ago in Europe, the Church forbade usury--the charging of interest by money lenders--so people had to borrow money from Jews instead of Christians.
We often find that ethnic groups in different parts of the world who are considered to be different, who are often discriminated against and persecuted, whose property has been confiscated, who have been through seriously hard times again and again, who value education and help each other out in good times, often work a little harder at saving and investing money.
When other folks save for a rainy day, they make sure they have enough money to buy an umbrella if they need one. They like to have money, but they value other things more. So they go into debt to get their own apartment and a new car when they graduate from college, go out for a beer every now and again, instead of staying at home and investing the extra money they make.
I used to work with someone who was not Jewish, by the way. She bragged that after she had attended college on financial aid--grants and scholarships--she had some money saved up because she did not spend it all. I had just finished college, and although I lived very modestly compared to some of my friends, I certainly had not saved any money from my financial aid.
One day in the restroom, I saw a note taped to the sanitary napkin dispenser in which my co-worker said that she had lost a quarter in the machine, and could someone please return it to her. Now, if I'd lost a quarter there, I would not have taped a note to the machine with my name on it. I would have just thrown that quarter away, because some things are more important to me than money.
And that's why I'm not a billionaire.