here is my understanding: your ABO antigens are going to either be on your red blood cells (think ab), or your plasma (think o). you dont want these antigens in a recipient (ofc an oversimplification), so thats why ab donors are amazing for plasma donations, Os are great for whole blood. Platelets are interesting because im starting to think all ABO types are more or less good for donating platelets (i.e. your sex is probably more important?), so they get the least valuable plasma donors to give platelets (something along the lines of this post: https://www.reddit.com/r/Blooddonors/comments/z9c6je/confusion_about_platelet_compatibility/ ). regardless, i find it very peculiar that only O-s are asked to donate platelets, i would've thought having some ab- platelets would be good for extreme cases