It's because blood bags are typically pre-filled with a measured dose of an anticoagulant to increase how long the blood can be stored. If they're only partially filled there will be too much anticoagulant for the volume of blood.
I believe the most common anticoagulant used today is Citrate Phosphate Dextrose Adenine Solution (CPDA-1), which doesn't have a Wikipedia page, but it's mentioned on the page for the the older acid-citrate-dextrose:
However, I agree that volumes of blood per donation (which varies by country) are sometimes so big that it probably discourages many potential donors. It's possible that demanding such a large quantity from each donor reduces the total amount of blood donated.
I believe the most common anticoagulant used today is Citrate Phosphate Dextrose Adenine Solution (CPDA-1), which doesn't have a Wikipedia page, but it's mentioned on the page for the the older acid-citrate-dextrose:
https://en.wikipedia.org/wiki/Acid-citrate-dextrose
However, I agree that volumes of blood per donation (which varies by country) are sometimes so big that it probably discourages many potential donors. It's possible that demanding such a large quantity from each donor reduces the total amount of blood donated.