Why were the rules of purity not limited to Zen monasteries in China?

The rules of purity, or qinggui, were not limited to Zen monasteries because they were essentially a codification of traditional Buddhist priestly norms of behavior rather than unique inventions of the Zen school.1

Beginning in the Song dynasty, these regulations were established to govern conduct across all publicly recognized Buddhist institutions in China. While they are often associated with Zen, they were applied to the residents of all authorized monasteries, regardless of whether those institutions were affiliated with the Zen school or other Buddhist traditions.1