This is because from the emperor Jiaqing (r. 1796-1820) onwards, the country went into a continuous decline, and the later emperors did not share Qianlong's passion for art. Thus the habit of collecting gradually lapsed.
因為嘉慶之後，國力日漸衰微，後來的皇帝又不像乾隆這麼雅好文物，因此收藏風氣日漸式微。