In the post on beans and kitniot I mentioned crop rotation as a possible reason for the fear that beans and grain may become mixed up. I just found an interesting article that explains why this was only a concern for Ashkenazim but not for Sephardim:
But there is a good reason for the emergence of this custom in specifically Ashkenazic lands. Ashkenazic communities, and the custom of kitniyot, originated in the temperate regions of northern France and the Rhineland. The climate there differed from the Mediterranean climate of Sephardic communities in two key respects: its summers were far milder, and it rained all year around. Each of these elements produced a change in agricultural practices. The milder summer meant that one could harvest twice each year, once in the winter and once in the summer, thus making the land more productive. Specifically, it was in the temperate regions of northern Europe that the three-field system of crop rotation was developed.
When the same field is used to plant a type of grain one year and a type of legume the next year, there are inevitably stray stalks of grain growing amongst the kitniyot. Under the two-field system of the Mediterranean and Middle East, a field would be planted one year and left fallow the next. Thus, legumes were not planted as frequently, and even if a particular field was changed from grain to legumes, there would be two years’ separation between the crops. The admixture of standing crops would have only been an issue in Ashkenazic lands.
A second difference pertains to the sheaving of harvested grain. In semiarid lands, crops were harvested in the early summer and gathered in the autumn, before the start of the rainy season. Thus, Shavu’ot, in the early summer, is a harvest festival, while Sukkot, in the early autumn, is a gathering festival. Because there was no rain expected during this interval, the harvested grain could be left in sheaves or heaps out in the field where they grew; there was no concern that rainfall would ruin them. This was not the case in Ashkenazic lands, where rain could fall any time throughout the year. There, special structures had to be built for grain storage near the fields. Since there was more than one harvest throughout the year, the same granaries were used for different crops—grains during one harvest and legumes during the next. Thus, the very structure where wheat had been heaped a few months earlier would be home to a heap of legumes just a few months later. Once again, this concern was completely absent in Sephardic lands.
(Elli Fischer: Why Sephardim Eat Kitniyot but Ashkenazim Don’t)