- Kitniyos are often grown in close vicinity to the five grains (wheat, oat, spelt, rye and barley). As such, it was not unusual for a small amount of one or more of the five grains to be intermingled with kitniyot. Thus, it was possible that one eating beans or rice on Pesach could inadvertently eat actual chametz
- Kitniyos can be easily confused with chametz for several reasons. Raw kitniyot resemble the five grains in appearance. Furthermore, kitniyot are processed in a similar manner to the five grains. For example, mustard seeds are threshed and winnowed in a manner similar to grains. Finally, kitniyot can be milled into flour, made into dough, baked into bread or cooked into a porridge that may resemble chametz. Because of the similarities between kitniyot and actual chametz, the rabbis feared that people may mistakenly believe that if they can eat kitniyot on Pesach, they can also eat chametz on Pesach.
Both of these 2 reasons no longer apply today in any way shape or form. Again from the OU:
In our contemporary society, the original motivation for avoiding kitniyot is no longer relevant. Hundreds of years ago, when the custom was first instituted, there was concern about people confusing legumes and grains, and thereby unwittingly eating chametz on Pesach. But what would our ancestors say to the unbelievable variety of kosher-for-Pesach-food items resembling chametz that are ubiquitously available today? Until about fifty years ago, Pesach fare was limited to mostly chicken, eggs, potatoes, and matzah. Nowadays, one can dine on kosher l’Pesach cereal, pizza, pasta, lukshen kugel, cookies, cake and almost anything else we eat year round. Is there any benefit then to maintaining the minhag of not eating kitniyot?The only justification offered is tradition, a link to Jewish history. IMHO, this is one of the problems with Orthodox Judaism. The religion has become fossilized, חדש אסור מן התורה, it has a very hard time adopting to the modern world. Tradition is great, but when it directly impacts people negatively it loses it's charm.