DI is a RARE disease of water imbalance, caused by lack of vasopressin, a hormone secreted by the pituitary gland, that helps the kidneys manage water content of the body. DM (of all forms - there are 3) is very common (primarily in form of Type 2 DM) and is related to lack of BG (Blood Glucose) regulation; the underlying cause of this lack of regulation is lack of insulin production (Type 1 DM), lack of insulin sensitivity by the cells of the body (Type 2) or impairment of insulin production due to disease of the exocrine pancreas (Type 3c, aka pancreatogenic DM).

The early association of DI and DM was due to the common symptoms - polyurea (excessive urination) and polydipsia (excessive thirst). DI, Diabetes INSIPIDUS, got it’s distinguishing name because the urine of these patients is INSIPID - colourless, odourless, clear, unlike that of DM patients, who’s urine is dark yellow, sweet, pungent, cloudy. Of course - if properly managed, those characteristics go away.

But in the early days of “medicine”, none of what we
know now was understood - DI patients showed up with virtually the same symptoms as DM patients, so they got “lumped in” with them..
(...https://www.google.co.in/url?cad=rja&cd=13&esrc="https://edurev.gumlet.io/q=&rct=j&sa=t&source=web&uact=8&url=https://www.medicinenet.com/diabetes_insipidus/article.htm&usg=AOvVaw3Lh-BG7D8xhvoqeHAOZeUL&ved=0ahUKEwjpgI6_s4nYAhWJRo8KHX8RBVUQFghkMAw)