It seems odd the more people of a certain blood type would sustain injuries requiring blood, or that people of a certain blood type would care more about blood donation So if the people who heard about blood banks and were interested, and the people who needed them are nearly randomly distributed in the population, I would expect the demand vs supply of each type to average out to the same figure.

However, I have heard blood banks ask specifically for people with rare blood types to donate, so it would appear that this theory is wrong. Alternatively, there is an equal shortage of all types, and someone in marketing thought that the specification would attract more people. (Even though if I was going to use the dark arts to make a group more likely to come, I would target the largest one)