Why is negative o blood type the rarest blood type?

Oddly enough, type O blood in the U.S. is not by itself the rarest blood type. Actually O+ is one of the most common. But because O is the recessive of the A-B-O gene group, and also Rh- is recessive in it's gene, the combination makes it extremely rare. Roughly 85% if people have the Rh+ allele, and roughly 1/3 people are O, but those are only O+.
The rarest is actually AB-, because 3% or less are this type. O- people are around 5% of population.
However because O- blood is the "universal donor" (will not cause immune reactions with any other blood type when transfused in) for hospitals and blood banks, it is always in short supply.