My first thought was -- at what point did the American government gain legitimacy enough that the power was taken from the British Crown to put it into its hands? During the Revolution, that's made clear later in the books, but what if they had lost the war? What happens when a colony throw out the imperial power and dissolves into anarchy? Who does it then, when the knowledge could leak out?
And above all else -- what happens when revealing this to the government officials results in disbelief or the decision to act otherwise than uphold the masquerade? The whole point of it is to keep things secret. Admitting people to the secret because they merely happened to rise to power -- or inherit it, come to think of it, a hereditary monarchy would just as much a problem.
However much it is the duty of government to protect its citizenry, when that involves a massive conspiracy to conceal something, that's not something that will last for decades and centuries. Unless it's the government of the people inside the masquerade, not the mundanes. A secret conspiracy would work, because it could recruit for like-minded individuals, but anyone who comes along is weaker.