Why is there no C~Z blood type?

This should start with the origin of ABO blood group. 1900, K. Landsteiner, a researcher at the Vienna Institute of Pathology, found that the failure of blood transfusion was due to the aggregation of other people's red blood cells caused by some people's serum, but this did not happen in some groups. The next year, he collected blood samples from his colleagues, tested red blood cells and found that they had two antigens, which he named A antigen and B antigen respectively. Blood with antigen A is called type A, blood with antigen B is called type B, and blood without both antigens; It is called Type C or Type Zero (later renamed Type O). So far, there are two subtypes of type A, and the others have not been found in this range. Of course, there are other blood types, such as Rh, MN and Xg. So there is no blood type in the back.