It would be extremely troublesome for the Celestial Demon Sect to occupy their Earth Realm and convert it into a place suitable for Demons to cultivate. Even if the Celestial Cloud Sect were wiped out, the conversion process would still take a very long time.
In contrast, it would be much easier for them to occupy the Celestial Demon Sect's Earth Realm and convert it into a place suitable for their own cultivation.
The reason lay with the Spirit Veins.
A place suitable for Demon cultivation needed both Demon Veins and Spirit Veins, and the Demon Veins had to be of a higher grade than the Spirit Veins. A place suitable for them, however, only required Spirit Veins.
Therefore, when they occupied a Demon's territory, they could transform the cultivation environment by destroying the Demon Veins. But when Demons occupied their territory, they would have to create a Demon Vein where a Spirit Vein already existed.
Destruction was far, far easier than creation.
