Because the 8th son was a member of a party and was so powerful that he felt threatened.
The Kangxi forty-seven years (1708) on November 14th summoned the Manchu and Han civil and military ministers, so that all the people in all the ahjusshi in the choice of a person for the new Prince, said to choose the Crown Prince in all the ahjusshi, must listen to the advice of the people elected.
Who thought the development of events did not follow his expectations. Tong Guowei, Mazi, Alinga, Orendai, Kui Xu, Wang Hongxu and other important court officials, led by the joint declaration of Yin Cheng as the Crown Prince, so that Kang Xi was surprised.
Kang Xi hated party camping, and at that time, he was still in his prime, looking at the dynasty officials and the eight party's power can not not worry about their own throne, so this is all in the eyes of Kang Xi, the heart of the great dissatisfaction.
He once said: "the second brother rebellious, repeatedly lost hearts; Yin Cheng is repeatedly knot hearts, this person's risk, a hundred times more than the second brother also". So in the crowd recommended Yinzheng as heir to the throne, Kangxi not only angry, but also ordered the imprisonment of Yinzheng, thereby combating the "eight party".
Expanded:
The main reasons for the failure of the Eight Masters' Party:
1. of exposing their group's goals and strength. "Snipers and mussels are fishing," and what the Eight Masters Party did was not fishing.
2. Know your enemy and know yourself. The eight masters party has not been able to see who their real opponent is, perhaps the four masters are too good at disguise, the four masters have been in front of Kangxi to play the role of a good son and a good brother, he hid himself too well in the second abolition of the crown prince only after the beginning of the eight masters began to start the fight.
There is a painting preserved in the Palace Museum called Killing a Tiger in Plainclothes. The plainclothes man was hiding to watch two tigers fighting, one tiger died and the other was exhausted and wounded, when he went over and killed the other tiger with a knife.
Here, the party of eight masters, which lasted for 20 years, was finished, and there was no more drama on the stage of history, and the final victory was won by the Fourth Brother, the Yongzheng Emperor.
Baidu Encyclopedia - Aixinjueluo Yinzheng