The construction of the Necromantic Space still required many steps, and to prevent elemental creatures from invading the Space Fragments, the portal opened by Chen Yu was not very large. Pumping air and water into the Space Fragments took some time, so after assimilating the Space Fragments into the Amethyst Skull, he set it aside.
As for establishing the Mingyue Society and gaining his own power base, Chen Yu certainly gave such orders to Tsuchimikado Yoshiharu and Kyuuzai Ryousen, but in reality, he wasn't enthusiastic about it. The purpose of having the two of them set up the Mingyue Society was simply to ensure he had manpower available for similar situations in the future.
For a Necromancer, the thoughts of the living are too complex. Rather than taking the time to subdue subordinates and train enforcers, it was much simpler and more convenient to create a couple of corpses and turn them into Undead Servants.
