The thing is, perhaps the researchers were getting more and more coincidental evidence that the treatment didn't actually work ('I snuck some out of the lab and gave it to my whole family, but it didn't really seem to make much difference').

At that point, it looks much better on your CV to say "we made this miracle treatment, but the FDA put up hurdles to us getting it to market" rather than "we spent many years but the treatment we made didn't work".