AB- is the rarest blood type in the U.S. They don't have much need for it because it is rare. They like to have the rare types available on call just in case it is needed. Whole blood has a limited shelf life.
If you have a common blood type such as O+ or A+/- they are more than eager to take your blood regularly. If you are the Universal donor O-, they want as much of your blood as they can get.
AB+ while not as rare as AB- is also not very common and also not highly in demand. The other part about AB+ is that it is the Universal recipient while O- is the Universal donor.