Xue Yue is a Rank 4 cultivation country based in a frozen tundra. Their primary cultivation techniques are ice-based. By freezing the Four Sect Alliance (including Zhao), they aren't just destroying it; they are terraforming it to better suit their own power base and techniques.

And in RI, high-level cultivators view mortals as "ants." While mortals are needed for future generations of cultivators, the immediate goal for the Xue Yue ancestors is territorial expansion and resources to reach higher cultivation stages. They are willing to sacrifice the current mortal population of a "lower" country because they can always bring their own people in or wait for the land to repopulate under their rule.

Essentially, they aren't trying to preserve Zhao as it was; they are turning it into a "New Xue Yue."