Kui, Shang Yang, and Shen Buhai were all Legalists. Why was only Shang Yang successful?

First, let me point out an error in your question. Among the three representatives of Legalism during the Warring States Period: Li Kui, Shang Yang, and Shen Buhai, only Shen Buhai's reform in Korea failed, while Li Kui's reforms in Wei and Shang Yang's reforms in Qin were both unsuccessful. Very successful. From a certain perspective, Li Kui's reform has a higher historical status than Shang Yang's reform. Therefore, I will analyze the reforms of these three people below.

First, let’s take a look at Li Kui’s reforms. We know that everything is difficult at the beginning, and Li Kui was the person who started the trend of reform in the Warring States Period. Li Kui's reforms can be roughly divided into four items: abolishing the hereditary system of nobility, abolishing the well-field system, proposing the concept of governing the country according to law, and establishing Wei Wuzu. The content of Shang Yang's reform that later took place in the Qin State was similar to that of Li Kui. After absorbing the reform experience of his predecessors, Shang Yang incorporated his own unique Legalist ideas and changed the Qin State in less than ten years. The country has been poor and weak for a long time.

Let’s take a look at Shen Buhai again. The biggest difference between Shen Buhai and Li Kui Shang Yang and others in terms of reform is that Li Kui Shang Yang's ideological focus is on the "rule of law", that is, governing the country according to law. However, Shen Buhai's thoughts focused on "curing with techniques". The meaning of this technique is that the monarch of a country needs to use certain techniques to control his ministers.

You can understand this as a technique to control the officials. But the problem is that Shen Buhai's technique relies too much on the quality of the monarch. When Shen Buhai carried out the reform, his monarch Han Zhaohou was a relatively capable monarch, so Korea's reform achieved some results in a short period of time. But after the death of Han Zhaohou, his heirs no longer had the ability of their predecessors, so they wiped out all the wealth they had managed to save after the country became rich and powerful.