Qin Chongyang's intention to not inform everybody was, of course, trying to protect Qin Wushuang. Regardless, Qin Wushuang was a newcomer. If his publicity had flourished, even within the united atmosphere of the Qin Clan, most likely it would evoke unnecessary internal problems.
After all, it would be a great stimulation to other core disciples if he allowed Qin Wushuang's publicity to surpass all the youngsters of the Aspiring Throne Mountain Qin Clan.
It would not have mattered if it were positive news. He was afraid of evoking different types of negative feelings that could cause an imbalance of attitude or changes in mood. Certainly it would influence the morality of everyone.
The disciples of the Qin Clan would still accept Qin Wushuang slowly beginning with his existence. Everyone would need to gradually become familiar with Qin Wushuang's exceptional abilities.