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.

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.
The reason why Crown Prince Li Chengqian was deposed was related to the palace coup he had planned. According to records, during the reign of Emperor Taizong, the fight for the position of Crown Prince was extremely intense. Li Chengqian was Taizong's eldest son, but he was worried that his position as prince would not be guaranteed, so he plotted to force the palace to rebel in the 17th year of Zhenguan. As a result, the matter was exposed, and he was deposed as a commoner and exiled to Qianzhou. The failure of the coup had affected the fate of Li Chengqian and his followers. The epitaph's record of 'the Lord was humiliated before, and the minister was deposed after' may have implied this incident. Although Li Chengqian was deposed, the one who finally ascended the throne was Tang Taizong's ninth son, Li Zhi.
The main reason why Prince Li Chengqian was deposed was that he had lost his virtue and participated in the rebellion. His conduct was corrupt, and he lost the trust and love of his father and his subjects. He indulged in wine, sex, and pleasure, wasting the country's financial and material resources. In addition, he also befriended some crafty people, listened to their instigation, and betrayed his father and country. Such behavior made Li Shiming think that he was not suitable to be a qualified heir. Therefore, Li Shimin decided to abolish Li Chengqian's position as Crown Prince.
He could find many novels about the deposed Crown Prince, Li Chengqian. One of them was " The Rebirth of Li Chengqian, the Crown Prince of the Tang Dynasty ", which told the story of a modern otaku who traveled to the 17th year of Zhenguan in the Tang Dynasty and became the Crown Prince Li Chengqian. Crown Prince Li Chengqian was facing the crisis of deposing the Crown Prince. The people around him also had ulterior motives and constantly encouraged him to rebel. The other novel was " Tang Dynasty: Starting from the Deposed Crown Prince's Registration ". The story was about the deposed Crown Prince Li Chengqian awakening the registration system after he was exiled to Qianzhou. Through his efforts and contributions, the deposed Crown Prince was praised and respected by the people. There was also a novel called " The Great Tang: Self-Deposing the Crown Prince at the Beginning ", which told the story of a 21st-century teenager who accidentally transmigrated to become the Crown Prince of the Great Tang, Li Chengqian. He had awakened the system and requested to depose himself as the Crown Prince, which shocked the imperial court. The above was the answer based on the search results provided.
The reason why Crown Prince Li Chengqian was deposed was related to the palace coup he had planned. According to historical records, Prince Li Chengqian plotted to force the palace to rebel in the 17th year of Zhenguan, but after the matter was exposed, he was deposed as a commoner and exiled to Qianzhou. This coup might have been because Li Chengqian was worried that his position as Crown Prince would be lost, and Emperor Taizong's other son, Li Tai, was deeply favored by the Emperor, forming a threat to the Crown Prince. Thus, Crown Prince Li Chengqian's coup led to his fate of being deposed as a commoner.
The main reason why Prince Li Chengqian was deposed was that he had lost his virtue and participated in the rebellion. His conduct was corrupt, and he lost the trust and love of his father and his subjects. He indulged in wine, sex, and pleasure, wasting the country's financial and material resources. In addition, he also befriended some crafty people, listened to their instigation, and betrayed his father and country. He also plotted to assassinate his brothers and ministers, overthrow his father's rule, and seize the throne. Such behavior caused him to lose the qualification to be the heir and was eventually deposed as the Crown Prince.
Empress Zhangsun gave birth to three sons, namely Li Chengqian, Li Tai, and Tang Gaozong Li Zhi. Li Chengqian was the eldest son of Empress Zhangsun and Li Shimin. He was made the Crown Prince, but he was demoted to a commoner because of his rebellion. He finally died two years after being demoted. Li Tai was Li Shimin's fourth son. He was ordered to participate in the compilation of literature because he liked literature. He died at the age of 35. Li Zhi was Li Shimin's ninth son, who later became Emperor Gaozong of the Tang Dynasty. He died at the age of fifty-five. Empress Zhangsun also gave birth to four daughters, namely Princess Changle Li Lizhi, Princess Chengyang, Princess Jinyang Li Mingda, and Princess Xincheng. Empress Zhangsun had an important influence on Li Shimin's imperial career, especially after he became the emperor. She helped Li Shimin create the Zhenguan Rule. Li Chengqian became absurd after Empress Zhangsun's death. He indulged his desires and even rebelled. He was eventually deposed.
During the reign of Zhenguan, Li Chengqian was deposed as the crown prince. According to the documents provided, Li Chengqian plotted a coup in the 17th year of Zhenguan and was deposed as a commoner by Emperor Taizong of the Tang Dynasty. This move was to prevent the tragedy of the three brothers, Li Chengqian, Li Tai, and Li Zhi, similar to the incident at the Black Tortoise Sect. In order to protect his three sons, Taizong exiled Li Chengqian to Qianzhou and stripped Li Tai of his title, demoting him to the Duke of Donglai Prefecture. In the end, Taizong made his ninth son, Li Zhi, the Crown Prince, and Li Zhi ascended the throne. As for the specific reason why Li Chengqian's position as Crown Prince was abolished, the documents did not mention it.
There were many reasons why Li Chengqian was deposed as the Crown Prince. First of all, his conduct was corrupt, and he had lost the trust and respect of his father and his subjects. He did not obey his father's orders, did not respect his teachers, did not care about political affairs, indulged in wine, sex, and pleasure, wasting the country's financial and material resources. He had also befriended some crafty people and listened to their instigation, betraying his father and country. Secondly, Li Chengqian's rebellion violated his father's bottom line and endangered the country's security and stability. He attempted to assassinate his brothers and ministers, and even wanted to overthrow his father's rule and seize the throne. Such behavior forced Li Shimin to depose him as the Crown Prince and exile him to other places. In general, Li Chengqian's misbehavior and rebellion were the main reasons for his dethronement.