Allegiance to the movement

The secret police is the crucial institution of a totalitarian regime. They are made up of the elites of the movement and they serve as the enforcers of the movement's values into the state. Whether Gestapo, NKVD, or Revolutionary Guard, they enact the will of the Leader. The secret police is to a totalitarian regime as the military is to a non-totalitarian regime. It's the secret police that imposes the primacy of the movement onto the state and ultimately onto the population of the nation. 

The military could not impose the movement's values onto the state because the military must function according to a code of honor, and a code of honor goes against the logic of an absolutist movement. Only movement zealots would be willing to impose the Leader's rule onto the people against the will of the people. The military cannot justify murder of members of their own population. Only the secret police whose allegiance is not to the state nor to the nation but to the movement could justify such conduct.