The kidnappers had warned Soichiro against making a move and rescuing Sayu, or else, they would kill Sayu. Soichiro unwillingly gives the order to send word to the police department until Light convinces them that Kira was likely the one who killed the director. He has probably infiltrated the police force once again, and if word gets out that they will exchange the notebook for Sayu, Kira might kill them all.