From the Nature article:
Type O is the most common blood group, but stocks constantly run low because it is used in all emergency situations where there is no time to determine the patient's own group.
An additional pressure on type-O blood, particularly in the United States, comes from the mismatch between blood donors and recipients. Most US donors are Caucasian, amongst whom 45% of people are blood group O. But more than half of African Americans, and nearly all native Americans are O, and so require type-O blood.