While it's tempting to look for one big reason, the real explanation is likely to be a combination of many little ones, such as:
- Historical emphasis on learning: Jewish religious education has traditionally encouraged (male) literacy and (religious) learning from a young age. To an extent this was natural selection: the Pharisaic tradition of education succeeded partly because illiterate Jews were more likely to become non-Jewish over the years.
- Gravitation to urban areas: religious quorum requirements and dietary laws, combined with historical restriction on land ownership, ghettos and recent migration history, have resulted in Jews being a disproportionately urban population, placing them near the centers of learning. Again there was an element of natural selection at work: rural Jews were far more likely to lose their Jewish identities.
- Geographic distribution: Jews were disproportionately present in Central Europe (Germany, Austria, Hungary, etc) during its period of economic and cultural dominance, and migrated to the United States just as it was entering its own period of dominance. The timing of the migrations from Eastern Europe also meant that Jews (like Italians) disproportionately settled in the more scientific, coastal cities rather than in the industrial heartlands of the US.
- Effects of discrimination: as Jews were gradually permitted into European society, some fields such as science (which was relatively impartial and meritocratic) were far more attractive to Jews than other areas (which often maintained strict religious or class-based entry barriers). Discrimination within science (such as Jewish quotas and a difficulty in getting tenure) may also had an unintended positive effect by 'forcing Jews to work harder', though it's not obvious to me how.
- Minority status and 'clannishness': like the Chinese in SE Asia, the Parsis in India and the Indians in E Africa, Jews became a middleman minority in their adopted countries, partly due to (and in turn reinforcing) strong family ties and 'clannish' behaviour such as endogamy.
- Secularization and liberalism: the secularization associated with the Haskalah led to an embracing of progressive, modernist values, science among them. Prior to the Haskalah, the Jewish world produced very few thinkers of international significance, despite its strong emphasis on (religious) education.
- Economic success: since the mid 20th century, the Jewish population has become disproportionately middle or upper-middle class, resulting in better than average education and other opportunities.
- Social capital and expectations: together with economic success, many Jewish communities also developed strong cultural expectations of academic achievement, together with social networks that help enable those expectations (similar to many Asian communities).
Some people also attribute Jewish success to Ashkenazi genetic differences but it's important to remember that Jewish scientific and cultural excellence is a very recent thing. Intelligence tests performed at the begining of the century didn't show any differences between Jews and non-Jews; likewise, Sephardic Jews from the 'right' areas are as successful as Ashkenazi groups (e.g. Italy's tiny Sephardic population of <50,000 boasts 4 Nobel Winners and countless other top academics). In many ways, the Nobel Prize covers precisely the period in history where Jews were most successful.
It's also worth remembering that the proportion of Jews among US PhDs has declined in recent years, as has spending on higher education in Israel. Combined with the increasing levels of research in Asia and elsewhere and the fact that the Nobel Prize is a reflection of research performed 30-50 years ago, it is certain that the proportion of Jewish Nobel Prize winners will decrease significantly in the not-too-distant future.
