There can be many reasons, you decide which one(s) you pick:
1. Jews have the better genes (racial superiority over non-Jews).
2. Jews raise their children diffrent than non-Jews, in a way which leads to a better brain development (social reasons).
3. Jews are the chosen people and blessed by god, as written in jewish holy books (religious reason).
4. Chess is an important part of Jewish culture, so that every child learns it at an early age (cultural reason).
5. Jews have more free time to spend for such things as chess.
6. It's all coincidence.
7. Whatever.
Fischer's father, Paul Nemenyi, was a Hungarian Jew. look at his photo:
http://en.wikipedia.org/wiki/Paul_Nemenyi
wow.