The Qianlong Emperor lived too long, causing his sons to die one step ahead of him. In the later years of the Qianlong Emperor, not counting the sons of the successors, there were only four sons hanging under his name: the Eight Brothers Yongxuan, the Eleventh Brother Yongxuan, the Fifteenth Brother Yongxuan, and the Seventeenth Brother Yongxuan, and the throne could only be selected from these four people.
The old eight feet had a disease, and the Qianlong Emperor could not be pleased, and was directly excluded. The Qianlong Emperor also looked down on the old seventeen who did not learn and did not have any skills. The Qianlong Emperor could only choose between the old eleventh and the fifteenth oldest, and the winner was the old fifteenth, that is, the Jiaqing Emperor, who changed his name to Yuanyan.

Why didn't the Qianlong Emperor choose Yong Yao?
In the case of the early death of the second sons born to Empress Fucha, Yong Lian and Yong Chun, both of them were out of the house, and neither of them took advantage of this. However, unlike the eldest sons of other dynasties, the Qing Dynasty generally adhered to the principle of establishing a virtuous person and not standing a long line, depending on who was more capable and more suitable to be an emperor.
When it comes to talent, Yongyan is not as good as Yongyan. Yong Yao's calligraphy was famous at that time, and the four major calligraphers of the Qianlong Dynasty, Yong Yao, were juxtaposed with Tie Bao, Liu Yong, and Weng Fanggang. Yongyan's words can also be read, but they can't be compared with Yongyan.
In addition to his good writing, Yong Yao's literary achievements are also above the fifteen brothers. Yong Yao's works, such as the "Essays of The Curse of Jin Zhai", the "Canglong Collection", and the "Book of the Law of the Curse of Jin Zhai" are all his works, which were circulated at that time.
Among the princes, the Sinicization of Yong Yao is very obvious, and you can see his nickname - The Master of the Jin Dynasty, the Number Mirror Spring. If you don't know his identity as an imperial brother, outsiders still think that he is a literary scholar in the Wuthering Heights Forest.
And these two points determined that his status in Qianlong's heart was not high.
First, the Qianlong Emperor was familiar with history, and when his sons competed for the throne, talented princes were often losers. For example, Cao Pi squeezed out of Cao Pi (Cao Pi himself is also everyone). Talented sons who became emperors, such as Emperor Xiao Gang of Liang jianwen, Emperor Xiao xuan of Liang Yuan, Meng Chang, the lord of Later Shu, Li Yu, the lord of Southern Tang, and Zhao Yao, emperor Huizong of Song, did not have a good end.
The Third Great Master of the Qianlong Emperor, Yin Zhi, was recognized among the Kangxi princes, compiling the "Origins of the Law Calendar" and "Integration of Ancient and Modern Books". But because of this, Kangxi would not choose him, first choosing the second elder Yin, and then the old fourth Yinchen (Yongzheng Emperor). Both the Kangxi Emperor and the Qianlong Emperor knew that too talented people had a weak personality, could such a character be suppressed in the official arena?
Second, the Qianlong Emperor himself sinicized and often played in Hanfu in the palace. However, he opposed the pro-Noble Sinicization of Manchuria, believing that this would lay the foundation for the fall of the Qing Dynasty. The Qianlong Emperor often demanded that Manchurian nobles should practice riding archery and Manchu hard, and not learn the "frivolity" of Han chinese doctors. But anyone who learns from the Han dynasty and claims to be the master of a certain house and the master of a certain room will be scolded by the Qianlong Emperor. Both disadvantages of Yongyao were accounted for, and the Qianlong Emperor was unlikely to choose Yongyao from the perspective of maintaining Manchurian rule.
The Qianlong Emperor did not choose Yong Yao, and there was another ridiculous reason - Yong Yao was a rare iron rooster prince in the Qing Dynasty, and he did not pull out a dime. Yong Yao was like his nephew Emperor Daoguang, who spared his wealth as his life, and let him spend one or two pieces of silver, it was better to kill him...
Yong Yao was not forced to become an iron rooster because of his shyness, but took pleasure in being an iron rooster. Prince Cheng's mansion had a small vault that outsiders could not enter, and as long as Yong Yao earned money, he would deposit the silver in the small vault. Accumulating over time, Yong Yao also saved 800,000 taels of silver, which was not a small amount at that time. But Yong Yao would rather let the silver grow white hair than use one or two pieces of silver. Without silver, what about eating? Yong Yao takes materials nearby, and eats whatever he can. He had a beloved horse dead, and Yong Yao was very sad. Yong Mao expressed twelve points of respect for this beloved horse: no one in the house was allowed to eat anything else, and today all of them ate horse meat.
The hall became the prince's mansion and became a slaughterhouse, and everyone was busy killing horses. Someone cuts horse meat, someone sets up a cauldron...
When the news spread, he became the laughing stock of the officialdom, and he killed his father, the Qianlong Emperor, and scolded Yong Yao to the point of "horse" bloodshed.
There is another trouble. Yongyao's Fujin Fucha clan was of noble origin, and her aunt was the Qianlong Emperor's beloved Emperor Xiaoxianchun of the Fucha clan, and her father was Fu Heng, the first assistant. But after marrying Yong Yao, Fu Cha clan understood one thing: the history of her eating and drinking spicy food and drinking at her mother's house was gone forever.
Don't talk about eating mountain treasures and seafood, big fish and meat always come to a few plates. Sorry, none. Of course, the husband can't let his wife go hungry, and the food always has to be eaten. The meal was brought up, and Fucha wanted to cry without tears: small pickles, crystal clear porridge.
You will say that Fu Jin brought a lot of dowry from his mother's family, and it is always okay to spend his own money. There was no drama at all, Fu Jin had just married, and her dowry had all been looted by Yong Mao, hiding in a small dark room.
Because of this, the Qianlong Emperor did not scold Yong Yao less and let him be generous. What kind of system does the prince become if he is miserly like this? But Yong Yao's literati temper could not be changed, and the Qianlong Emperor could only abandon him.
Putting aside Yong Yao, the only thing that the Qianlong Emperor could choose was the old fifteen Yong Yan who looked relatively thick. Yongyan was secretly appointed as his successor by the Qianlong Emperor as early as the thirty-eighth year of Qianlong (1773). In this year, Yong yan was 14 years old and Yong Yan was 22 years old. Although from the perspective of time, the Qianlong Emperor chose Yongyan slightly earlier, if Yongyan lost the Sacred Heart, the Qianlong Emperor could secretly replace him at any time.