The historical reasons for this are well known and have been discussed several times.
1. High rate of literacy among Jewish male population. Men were traditionally expected to read and study the Torah while women did the day-to-day work, which morphed into strong reading/writing skills as well as argumentation skills (the Kabbalah was meant to be debated). This skill set is directly applicable to the legal field.
2. Ability to speak several foreign languages. Jews were often between cultures and could communicate to other ethnic groups as well as their own. This makes doing business easier. In some sense, Jews were the first "citizens of the world."
3. Strong family structure. Even in Europe, Jews had an "us against the world mentality" in order to protect their unique cultural identity. Jews help other Jews. Jews hire other Jews. If one Jew gets hired to a non-Jewish firm, he will bring other Jews with him . . . but he only brings the talented ones.
4. Skills in certain fields. Even in Europe, Jews had become adept in certain profession by virtue of being banned form more "respectable" trades. There was already a long tradition of Jewish moneylending because it was considered immortal to charge interest. There are also more modern examples of this. In the US, it was considered a "dirty" business to run Vaudeville or operate a nickel theater. Jews were perfect for Vaudeville because their language skills allowed them to entertain several different immigrant groups. Jewish butchers did not mind running nickel theaters out of their delis and shyt. This eventually transformed into the Jews running the modern film industry. See:
http://en.wikipedia.org/wiki/Adolph_Zukor
5. For some Jews, the factors above were already in play when they grew up in Europe. Certain Jews arrived in America with a college education to start.
