Corn being able to be ground into flour, could mistakenly be mixed in with wheat, and mamesh made into chametz, so it was added to the forbidden list. At one time, so were POTATOES, because of the potato starch looking almost exactly like flour. But it did not last long, because it was SUCH a staple item in such abundant and cheap supply, that the people would have rebelled had it been assered. They would have had nothing to eat on Pesach. At least, this is what I was told.
