zahavasdad,
You can make flour out of alot things , that are not Chametz or Kitniyot
Correct. And some of these “flours” more closely resemble flour from grains that can be made into chometz, and we call the source material for those lookalike flours kitniyos. Because of the resemblance, Ashkenazim have a minhag of not eating kitniyos on Pesach. Rice made the cut and are considered kitniyos, potatoes did not make the cut, despite the fact that both can be made into “flour” and used to bake breadlike products. When new plants are introduced to a community that can be used to make “flour”, it is up to our Torah authorities to categorize them.
The ability to make flour is not the reason to make Kitniyot
No, but the ability to make flours that can be confused with flour that can become chometz is.