The main reason that you cannot donate blood is because it could be disastrous to give anticoagulated blood to someone who was already having uncontrolled bleeding.
Why you cannot give platelets, I'm not sure. I think it is probably the "you could bleed to death from a venipuncture" myth. It is like trying to get a law repealed, once the rule is on the books, nobody is going to try to get it off.
I'm not sure if this applies to organs or not. For most organ donations, you do not have to worry about bleeding to death!!! They are also well rinsed out before they are transplanted. You could keep yourself on the donor list because these are decided on a case-by-case basis.
Suggested Reading:
Stiff: The Curious Lives of Human Cadavers by Mary Roach