Why a secret Death Star cannot spread fear

The Tarkin Doctrine seeks to frighten subjects into compliance through overwhelming destructive power. It also depends on people informing on their neighbours to avoid annihilation, which requires widespread knowledge of the threat. Keeping the Death Star secret prevents that fear from spreading, much as concealing the Yamato class limits its value as a visible warning. Rebels have already chosen to risk death for freedom, so increasing the scale of destruction may leave that choice unchanged. The Empire also lacks an opponent with a matching Death Star whose retaliation would impose mutual restraint. The intended economy in personnel depends on fear doing some of the work of direct control.