This article on ABC News goes in to the history of haemophiliacs contracting Hep C and HIV via blood products. Clotting products come from hundreds of donors so sufferers were disproportionately affected by infected blood donations. 

While I’m glad to be able to donate blood at long last, I never really objected to not being able to because of the history of contaminated blood products. 

The current questionnaire focuses on whether someone has multiple sexual partners, regardless of gender or sexual preference, which I think is a significant improvement. But I understand why it took a long time.