Expanding a world was a tricky process, and it had different requirements depending on the type of power that it contained. Noah's ethereal darkness had pure potential as its fuel, so incredible feats spread his influence.
Of course, the amount of expansion depended on the nature of the feat. Slaughtering countless magical beasts or cultivators at his level wouldn't do much since he had already stepped into a superior realm. He needed to do more than that to experience substantial improvements.
Heaven and Earth's evolution and the current state of the world prevented Noah from doing as he wished. The rulers were benefitting from his incredible power and feats, and the weak sky stopped him from finding suitable targets.