They sell Potato bread with the Frankfurter or Hamburger Rolls
Only those Ashkenazic poskim who were part of the original kitnios g’zeira (about 600-700 years ago) were permitted to determine which foods would be part of the g’zeira; we cannot do it on our own.
Corn was not available in Europe. Its a totally american crop and was brought back to Europe after the Europeans came to the new world
Mustard is another example of the ever increasing chumralaztion of Kityot