I'm O+ and get fairly frequent reminders to donate. When I look for appointments there's mostly some in the next couple of days, and then a gap out 2 months or more. I'd assumed that meant they were opening up extra appointments close to time due to staffing or cancellations or something, hadn't thought that it might be about prioritising a needed group.
Also, the last time I went I was handed a card that I had the "Ro" subtype which is rare amongst blood donors but more common in sickle cell patients, amongst others, therefore much needed. Last I checked I think I was still CMV -ve too.
The person who mentioned "Du" - this means that they are Rh positive, but it is a weak version so normal testing might suggest Rh negative; hence it is important to know about. The "Ro" subtype is Rh positive for the main, D, antigen, but negative for two others, C and E, when more commonly people are positive for either or both.
So, to answer someone else's question about why is it A, B, O not A,B,C etc - The blood group antigens were indeed labelled A,B,C,D,E as they were identified. O is part of the system that includes A and B, O is the absence of both A and B, whereas AB is the presence of both. C,D,E antigens make up the Rhesus group. Then there are various other antigens that are less important to safe transfusion in most people, but if someone has a transfusion they may develop antibodies to them that would need to be taken into account if they needed another.
The reason the rules are so strict on preventing donations from people who have had a transfusion in the past are partly about reducing the risks of passing on new blood-born diseases that we don't know about yet.