I was looking through a plant catalog, and noticed that only one type of CANNA is edible. Why aren't other species edible also?
Being Zingerberales it may be hard to track that information down, as it could be either production of toxic or unpleasant tasting compounds or lack of useful features.
However, what I did find is that this species (Canna indica, also known as C. edulis) has large rhizomes, which are the main agricultural component, though you can also eat the flowers and seeds. C. indica has been extensively cultivated and most likely selected for rhizome size during its agricultural history. It would seem that the other species have much smaller rhizomes, so might not yield much that is worth eating, and so C. indica is the only species that is commonly eaten.