This was deliberately done by heavenly Emperor yuanchu. The heavenly court's attention had to be on other worlds at all times, not the human world. The human world was the core of the eight desolates world and also its back garden. Otherwise, there would be no need to set a threshold.
Even for other orthodoxies, they were only allowed to carefully select their disciples. They were not allowed to recruit disciples on a large scale, nor were they allowed to spread their teachings. They were not allowed to secretly control the dynasty. If they were discovered, even the allheaven immortal sect would be punished.
It was extremely important to maintain the purity of the mortal realm. Other than the development of the mortal realm, it was also necessary to prepare for the six paths of reincarnation. The reincarnation was still in the process of being born, and too much killing and sin would interfere with the formation of the reincarnation.