The fundamental reason why Duan Lingtian did not sweep through the various demon clans and slaughter all their powerhouses was his wariness of the human demon clan's so-called "ancestor."
Although the human demon clan was now almost entirely under Duan Lingtian's control following the escape of their clan leader, who had shattered a Blood Flame Escape Talisman to flee, this human demon clan did not include the clan leader, Yang Zhenxing, nor did it include the other Loose Immortal powerhouses of the clan!
After fleeing, Yang Zhenxing stood a very high chance of contacting that ancestor of the human demon clan—a being suspected to be above the Seventh Tribulation Loose Immortal level—and having that ancestor move against him. If Duan Lingtian remained in the Lower Realm of the Dao Martial Saint Land, even if he didn't stay within the territory of the human demon clan, that ancestor might still be able to track him down.
Chapter 2331