The problem they have with changing their stance on blood transfusions is that (1)people have died from following it and (2)they celebrated these cases and held them up as examples to be admired. Changing the policy would be a massive slap in the face to anyone who risked/lost their lives and would invite lawsuits from family members and loved ones. It would also be a huge black eye for the organization. If so much as a single media outlet ran a story on it, the backlash could be devastating.
They've just about allowed it over the years, by allowing almost every fraction in some combination or other, and by occasionally mentioning that it is a "conscience matter." I don't know if they'll ever be able to go further than that without risking litigation and bad press.