First of all, Zhu Qiyu abolished the post of Prince Zhu Jianshen, and the relationship between them was irreconcilable.
Originally, according to Sun's meaning, he could be proclaimed emperor, but the position of the prince must belong to his son. However, after Zhu Qiyu ascended the throne, he didn't keep the agreement. He not only abolished Prince Zhu Jianshen, but also appointed his son Zhu as the prince according to circumstances, that is, after Zhu Qiyu abolished Prince Zhu Jianshen, the contradiction between the two uncles was irreconcilable. In this case, Zhu Jianshen can't be Zhu Qiyu's stepson, and Zhu Qiyu dare not cross the line.
Second, Zhu Qiyu is still young, and there is still hope for having a son.
After Zhu Jianji's death, Zhu Qiyu was not very old. At this time, if he was lucky, he could have waited until the day when his son was born, because Zhu Qiyu could have had his own son if the restoration of Yingzong was not too fast. At the time of his death, there was already a pregnant concubine in his harem, and this concubine really gave birth to Zhu Qiyu's son, so Zhu Qiyu would never give up the opportunity to make his own flesh and blood a prince.
Third, if Zhu Qiyu adopts Zhu Jianshen as his stepson, he will only die faster.
Zhu Qiyu hasn't given birth to a son for a long time, and ministers are beginning to fidget. Even some ministers who originally supported Zhu Qiyu turned to support Zhu Qizhen. If Zhu Qiyu adopts Zhu Jianshen as his stepson, he will not only declare that he can't have a son in disguise, but also be suspected of stealing someone else's son. Ministers' anxiety will only get worse. Some opportunistic villains helped Yingzong seize the throne and his son, with greater profits and lower risks, and Zhu Qiyu would die faster.