Why is AB negative the rarest blood type?

1% or less

About 1% of the population has AB- blood. There are slight differences in the percentage from country to country but in the US and most other countries AB- is approximately 1%

Type AB negative blood is quite rare. In general, the percentage of this blood type in any population is less than 1%. This is due to the fact that type AB blood is the least common type of blood in virtually all populations and, furthermore, Rh negative blood is typically found in only 15% or less of any population. The exact percentage of AB negative blood varies among different ethnic groups and in different geographic regions. For example, the overall U.S. average is approximately 0.6%, the value in African-Americans is about 0.2%, and the value in Oriental-Americans, Hawaiians, North American Indians, and Australian Aborigines is virtually zero percent.