Thats the major difference between type 1 and type 2 diabetes
In type 2, often it can simply be the result of too much sugar in one's diet for the pancreas to properly deal with. Diet in that case CAN "cure" diabetes, if only because it was self inflicted in the first place (and is thusly normally attached to obesity).
But ya, type 1 is not curable with diet, and probably the majority of type 2 cases. In these cases diet has to be changed, but not to cure the individual, but to prevent them from dying. For instance, my mom cannot eat sugar. This is so she does not die.