First, Zhu Biao is the eldest son of Zhu Yuanzhang.
According to some official records handed down from the Ming Dynasty, Zhu Biao was the eldest son of Zhu Yuanzhang and his first wife, Ma Huanghou. According to the legal system since ancient times, it is natural for the eldest son to inherit the unification, so it is even more difficult for Zhu Biao's brothers to take away his crown prince. In addition, Zhu Biao, the eldest son, was also the eldest son of Zhu Yuanzhang and Ma Huanghou, so Zhu Yuanzhang attached great importance to Zhu Biao. Zhu Yuanzhang didn't have a son like Zhu Biao until he was 27. Although this is normal for us modern people, it is of great significance to Zhu Yuanzhang.
Second, the system of "sealing vassals" formulated by Zhu Yuanzhang has helped Zhu Biao to settle many obstacles for a long time.
When Zhu Yuanzhang made Zhu Biao a prince, he was thinking about how to help him stabilize the position of the prince, so Zhu Yuanzhang came up with the idea of "sealing princes". The so-called "vassal" is to give all the adult vassals in Beijing some local power and some military power, so that they can manage their own vassal towns well. But this "vassal" just sounds good, in fact, it is to drive these adult vassals out of the house. Zhu Biao's 26 brothers couldn't even come back to Beijing, and his younger brothers didn't have the strength to fight with Zhu Biao, which simply didn't threaten the status of Prince Zhu Biao. In this way, the status of Prince Zhu Biao will not be threatened by his brother.
Third, Zhu Biao is gentle, honest and talented.
Zhu Biao is not only gentle, but also attaches great importance to his feelings with his brothers. Whether he helped his younger brothers to take the blame when he was young or he helped his younger brothers to bear political mistakes when he grew up, Zhu Biao was the first to rush over and admit his mistakes. Plus Zhu Yuanzhang began to cultivate his political ability when he was only eleven years old in Zhu Biao, so Zhu Biao's ability as a prince still has leverage.
From this perspective, Zhu Biao was able to hold the post of Prince because he was the eldest son and was favored by his old father Zhu Yuanzhang. He is also a very regal person. Coupled with the greatest guarantee of the "vassal system" in the Ming Dynasty, no prince took Zhu Biao as the crown prince.