The custom of not eating kitniyot and rice on Pesach is first mentioned in France and Provence in the 13th century by Rabbis Asher of Lunel, Samuel of Falaise and Peretz of Corbeil. From there it spread to various countries and the list of prohibited foods continued to expand. Nevertheless, tthe reason for the custom was unknown and, as a result, rabbis invented at least 10 different explanations. For example, chametz sounds like chimtzei (= humus = chickpeas); if we allow kitniyot porridge, we will eat grain porridge because both are cooked in  a pot; rice and kitniyot are somethinges mixed with wheat. The large number of explanations for not eating kitniyot proves that no one knew the real reason. I believe that the original custom was to refrain from kitniyot on all festivals, not just Pesach, because kitniyot were associated with poor people, mourners, and Tishah b'Av. 

Whatever the original reason, Rabbi Samuel of Falaise (13th century), one of the first to mention it, referred to it as a minhag mahmaut taut (mistaken custom), Rabbi Yeruham (Provence, 14th century) called it a minhag shtut (foolish custom), and Rabbi Ya'akov Ben Asher (Toledo, d. 1343) said "it is an unnecessary humra (stringency), which is not followed."

Therefore, the main halakhic question is whether it is permissible to do away with a mistaken or foolish custom. Many rabbinic authorities - including Rabbi Abin in Talmud Yerushalmi, Pesachim, Maimonides, the Rosh, and the Ribash- ruled that is is permitted (and perhaps even obligatory) to do away with this type of foolish custom. Furthermore, there are may good reasons to do away with this particular custom. It detracts from the joy of the holiday by limiting the number of permitted foods. It causes exorbitant price hikes, which contradicts the legal principle that "the Torah takes pity on the people of Israel's money." It emphasizes the insignificant (legumes) and ignores the signficant (chametz from the five kinds of grain). It causes people to scoff at the commandments; if we observe this custom that has no purpose, there is no reason to observe the other commandments. And, finally, in Israel it causes unnecessary divisions between Ashkenazim and Sephardim.

haggadah Section: Motzi-Matzah
Source: Professor David Golinkin