There was no clear answer as to why Li Hong had been deposed as Empress. The search results mentioned some background information about Li Hong being made Crown Prince and Wu Zetian's power, but they did not provide any specific reasons or events. Therefore, according to the information provided, I don't know the specific reason why Li Hong was deposed as Empress.

The main reason why Li Chengqian was deposed as the Empress was because of his lack of virtue and his suspicion of rebellion. Li Chengqian's character was corrupt. He indulged in wine, sex, and entertainment, wasting the country's financial and material resources. He lost the trust and love of his father and his subjects. In addition, he also befriended some crafty people and listened to their instigation. Li Chengqian was also suspected of rebellion and attempted to assassinate Li Tai, but failed. These actions made Li Shimin think that Li Chengqian was not suitable to be the monarch, so he abolished Li Chengqian's position as the Crown Prince and exiled him to Qian Prefecture.
Li Hong was deposed as Crown Prince in the year of Xianqingyuan (656 AD).
In the works related to Legend of Zhen Huan in the Harem, although Yongzheng wanted to abolish the empress, he was unable to do so due to the constraints of his last words and edict. After Qianlong ascended the throne, Zhen Huan told Yi Xiu that she would never be promoted to Empress Dowager and could only be buried in the imperial mausoleum after her death. Although she was not deposed, her ending was tragic. She was imprisoned in Jingren Palace and lost her actual power as Empress. She broke with the Emperor and could not see him again in life and death. Even after the Emperor died, she was not qualified to worship him. She wanted to become Empress Dowager but failed in the end. Until her death, she only had the title of Empress.
From the reference materials alone, there was a related content mentioned in " The Love and Hate of a Fickle Emperor: The Abandoned Empress Becomes a Concubine." In it, the female protagonist was killed for his entire family. When they met again, she went from a maid to an empress and then to the Abandoned Empress, and was given to him as a concubine. On the wedding night, he sneered and ridiculed wantonly, but he did not elaborate in detail on the other reasons for becoming the Abandoned Empress. Therefore, the information that could be obtained about the Fickle Emperor was relatively limited.
'Who Touched My Disposed Empress' was a web novel. The plot revolved around the female protagonist. The female protagonist woke up from a different world and saw a hot scene. The previous owner of this body was extremely ugly and stupid. The result of the woman chasing the man was that her legs were broken and she was dragged out of the palace. She was conferred the title of Crown Princess by the imperial edict, and on the night of their wedding night, she was humiliated by the Crown Prince. After her rebirth, she was no longer the love-struck girl of the past. Later on, a series of events attracted the Crown Prince. The original novel of the TV series " The Unique Empress of Generals " was called " The Rebirth of the Poisonous Empress of Generals." The original plot was equally exciting. You can click on the link below to read the original novel.
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Prince Li Chengqian was deposed as a commoner because of his rebellion.
There were many reasons why Li Chengqian was deposed as the Crown Prince. First, he had colluded with Li Yuanchang, the King of Han, Du He, and Hou Junji, the son-in-law of Princess Chengyang, in an attempt to force the abdication of the throne. However, his plan had been exposed and he had been deposed. Secondly, Li Chengqian's character was corrupt. He indulged in wine, sex, and entertainment, wasting the country's financial and material resources and losing his duty as the crown prince. In addition, he abandoned his studies, disrespected his teachers, and even assassinated his own teacher. These actions disappointed Emperor Taizong of the Tang Dynasty and made him feel that he was not suitable to become a monarch. Therefore, Emperor Taizong of the Tang Dynasty decided to abolish Li Chengqian's position as Crown Prince and demote him to a commoner.
😋I recommend the following novels to you: 1. [The Empress of the Book Always Wants to Bully Zhen]: Transmigrated to become an empress, framed by the people around her, not favored. The story of the empress bullying the emperor. 2. "Three Thousand Beauties: The Empress Doesn't Serve the Bed": The story of an unloved queen who has status but is not happy. 3. "Sheng Chong Liu Gong: Black-bellied Emperor Leng Qing Fei": The harem struggle, the story mainly revolved around the deposed empress and the black-bellied emperor. 4. "Ugly Woman Turns Over: Taming the Silly Prince": Transmigrating to become the unfavored Di daughter of the General's Mansion, slandered by others. Later on, she wanted to make a comeback and taught the silly prince to become the empress. 5. [The Queen of the Cold Palace is too free. She transmigrated to become an unfavored queen of a country. After leaving the palace, she became a hotel owner and had many beautiful men chasing after her.] I hope you like this fairy's recommendation. Muah ~😗
The exact time when Li Chengqian, the Crown Prince of Joy of Life, was deposed as Empress was not mentioned, so I don't know the exact time.