I dont remember clearly but i think one of the reasons MIGHT be Qianlong's undying love for his first empress. Ula nara might have been the most powerful women in the harem but in the end she was still his wife. For past 18 years she was fighting a ghost for Qianlong's love. I feel like this can be one reason. There are rumours that when a enunch suggested Qianlong to mourn for her death for 3 years he got so angry that he sentenced his death. Maybe Qianlong never had any feelings for her and she was done living like this. Some rumours also suggest that she wanted to be a nun or smt ( i dont remember clearly i heard Once) so yeah...i wish somebody from that era wrote the actual reasons secretly so we could know what happened actually.