The good old "is unethical to do a double blind study". This one is by far the best invention of the Big Pharma PR departments, you basically cannot make the study that will prove for sure that their product is safe (and stick it to all those anti vaxxers once and for all) Talk about having your cake and eating it too...
Continued assignment of subjects to placebo is unethical, once there is good evidence to support the efficacy of the trial therapy.
Unfortunately (or fortunately) since there was such strong evidence to prove the efficacy, it became unethical for the continued assignment of placebo.