Perhaps they're harder to detect? They won't fly as high as ballistic missiles, so they'll stay below the horizon of ground-based detectors. They may also gen

Perhaps they're harder to detect? They won't fly as high as ballistic missiles, so they'll stay below the horizon of ground-based detectors. They may also generate less of a heat signature at launch so they may be less visible to satellites. Or, maybe the fact that they aren't ballistic and are steerable means it will be easier for satellites to figure out their course and that they aren't a threat. I'm just speculating.