The reason is that Kangxi understood that any emperor should have his own team. Even if the team's ass is not clean, someone should go to work, otherwise the emperor will be left to work alone, and he will not be exhausted. Even when Yongzheng became emperor, he offended too many ministers, who were unwilling to help at many critical times. Even sometimes these ministers are very willing to watch Yongzheng's jokes.
Although Kangxi was resourceful, he was generous and indulgent to ministers and bureaucrats. As for Kangxi, it is not that he can't find the right person, nor does he have to find his brother to save him. It is impossible that no one can be talented in a dynasty with both civil and military skills! Therefore, in the late Kangxi period, the court was corrupt, and ministers went to the Ministry of Finance to borrow money, but they still refused to pay it back. This is nominally a loan, but it is actually corruption, which is less risky than corruption.
Officials are still officials, and it is offended to recover the arrears. This is a great disadvantage to anyone. So even if Kangxi said that sealing the crown prince was such a big temptation for people who did this job, no one dared to take it. He has been wooing court officials and won't do anything to offend people. Kangxi tested Lao Ba to see whether he valued the country or personal interests. The result was disappointing and he had to settle for the second best.
May 4th Movement.
Because apart from Yongzheng, no eldest brother is willing to take this hard job. Everyone wants imperial power, and this process can't avoid the support of ministers. If you offend the minister, the throne will be farther and farther away.