First of all, not all Jews are rich. But if you're interested in knowing, there are two main factors.
A...Jews place a high value on education. I was watching "My Name is Earl" a while ago, and the whiny blonde ex-wife said something like, "I want a real doctor. A JEWISH doctor." The same can be said for Jewish lawyers, as that is also a stereotype. But the underlying value behind those stereotypes is education. Graduate degrees are required for both medicine and law. Education is probably the single most important factor. And of course, many non-Jews have these values too, but it's more common among Jews, as it is a part of the Jewish culture (and other cultures too...I'm only speaking about the Jews now).
B...Jews have been persecuted for a loooong time. When a group starts getting killed off, mandatorily converted to Christianity, or deported for their religion, the members of that group tend to band together to survive. Not all groups do that, but the Jews have largely chosen to band together in the face of adversity from other groups. So Jews have a proclivity to help each other. Mormons have a similar philosophy on helping fellow Mormons more (that is true, believe it or not, and e.g. Romney won Utah by overwheming margins).
So...combine the two things together and Jews tend to be better at making money, as educated Jews tend to help out other Jews, and mentorship counts along with education. But many, many other groups make more money overall, and the richest people in the world are not Jewish. And all of this is JMHO.