Presumably because he's immune to all attempts to suppress his power. Peter was able to remember everything by following Adam's instructions to focus on what was most important to him. That completely reversed the Haitian's handiwork.
Likewise, Adam scoffed at the pills not working for 30 years. Probably the pills didn't work on Adam at all, because he's immune to any attempt to injure and power suppression is injury to him.
They could have pulled a Highlander on him perhaps, which we presume should work, but maybe that was too barbaric for even The Company. Or maybe they tried it once and Adam just grew a new head with the precise neuron patterns required to preserve his memory.
What's notable is that Adam's blood can heal others, so it sets up a potential conundrum with the virus. Adam should be immune to it, but what if a damaged virus were 'cured' by his blood?
Is Nathan immortal now? If not, would something as small as a virus be, if exposed to Adam's blood? Or could a more targeted approach