Albinism, for one, is caused by wonky enzymes/protiens, as are many other conditions. I guess they don't want to go sticking something a bit wonky into someone who is already a bit screwed ('cause face it, you don't get a blood/marrow transfusion because you're in fabulous good health).

Not the most scientific description of what I'm trying to speculate on, but if anyone can translate my ramblings, the TD crew can.