As lord protector, why can Cheng Can Jin Yao be hereditary and Qin Qiong not?

Cheng He, a famous soldier in the Tang Dynasty, must be familiar to everyone. They joined the team of Li Shimin halfway, which made great contributions to the establishment of People's Republic of China (PRC). Therefore, when Li proclaimed himself emperor, both of them got high rewards, and both of them were named lord protector. But later, the successful lord protector can be inherited and passed on to future generations, but it can't.

According to historical records, it is not difficult to find that Qin Qiong fell ill after a long battle and did not make new contributions later. According to statistics, Qin Qiong has experienced more than 200 battles and suffered many injuries. When I was young, I came out when I was old. Every time he got sick, he said, "I have been a soldier all my life, and I have suffered many serious injuries. I have lost a lot of blood back and forth. How can I not get sick? " Since the ninth year of Wude and the Xuanwu Gate incident, Qin Qiong gradually faded out of the political arena.

Cheng is definitely weaker than in combat capability. He lives a long and healthy life. The 24 heroes of Lingyange, except the old fairy Xu Maogong, are probably him. Cheng is straightforward and rude in romance novels, but in fact he is a very smart and smooth person.

He was deeply trusted by Li Shimin, Emperor Taizong, and was promoted many times. After the establishment of the Tang Dynasty, Cheng was not a hereditary lord protector. In the year before his death, he was renamed Lord protector, and his descendants were hereditary. Since then, Cheng has been in charge of the military affairs of Youzhou, Yi, Tan, Ping, Yan and Guangxi, and almost ruled over more than half of China's military forces, which shows that the people of the world trust him.

In any case, the two Founding Fathers were treated well by Li Shimin, and their endings were satisfactory.