A. The question is the other way around: Why can’t Ashkenasic Jews (of European descent) eat Rice, which is not included in the Torah or Talmud’s definition of Chametz? The answer to that is, European Rabbis of more than a millenium ago, unanimously added Rice (beans, and corn) to the list of (Rabbinically) forbidden foods on Passover, because they too can be ground up as a flour and made into bread (we consumers of ready made processed foods may not realize the concern). The Rabbis of the Sefardic communities (Mediteranean lands) never adopted this Rabbinic decree, so it never became their tradition.
