1, according to custom, most people don't build toilets behind other people's rooms, even if everyone can't say why.
2. From the perspective of geomantic omen, the hall is a superior financial position and a place to worship ancestral tablets. How can it be defiled by the filth at close range?
3. From the perspective of building safety, the toilet is close to the back wall of other people's homes, and urine nitrate permeates into it, which is easy to destroy the wall.