Most donor blood is O+
O- is great because it is (almost always*) usable by anyone of any blood type, but only like 7% of people have it, so the overall supply is not very high and it’s typically reserved for emergencies and O- patients.
O+ can be given to most people because most people have O+ blood. Something like 30-40% of people have O+ and can thus receive O+ donations. All the Rh+ blood types can receive O+ blood too, which accounts for the vast majority of people. A+ is something like 25-30% of the population and B+ is like 10%. Altogether, O+, A+, B+, and AB+ make up probably about 75-85% of the population, and all of those people can receive O+ blood donations, thus the supply of O+ blood is the most-needed most of the time.
Obviously they also want as much O- as possible too. Hell, they want all of them all the time except AB type blood from females.
*Blood typing is actually a lot more complicated than A/B antigen and Rh factor, so some people with certain antigens need very specific donor blood or autologous blood that they donate themselves prior to a procedure or whatnot.
Oh it’s not exclusionary. They’ll totally still take your blood. It’s just in the lowest demand in general, and blood banks often tell AB females (both + and -) that they will reach out when they want/need blood donation, rather than telling those donors to come for donations as often as possible like they do for other donors. If you’re willing to donate, by all means keep donating. The blood banks can figure out what to do with it and all blood has its uses!