Personally, I think there may be the following reasons, all of which are romantic analysis.
First, Cao Cao was careful not to attack.
According to the Records of the Three Kingdoms, when Cao Cao rode with butch to chase Liu Bei and others to Dangyang Bridge, he only found Zhang Fei standing on Dangyang Bridge, and Zhang Fei looked confident and determined. According to the actual situation of the battlefield at that time, Dangyang Bridge was narrow and long, which was not suitable for underestimating the enemy.
In addition, Zhang Fei was not unprepared at that time. According to the description in the Romance of the Three Kingdoms, Zhang Fei sent 20 sergeants to gallop back and forth with broken wood on horseback, raising dust and creating the illusion of an ambush. Cao Cao in history was inherently sinister and suspicious. As we all know, Zhang Fei's actions inevitably made Cao Cao not doubt that there would be an ambush, so it is reasonable that he did not attack rashly.
Zhang Fei is also famous in the world. Cao Cao also heard Guan Yu say that Zhang Fei was brave and unusual, and saw that Zhang Fei and Lu Bu were unbeaten in a hundred rounds, which made him actually afraid of Zhang Fei from the heart. Dare not make a move, so Cao Cao didn't order war at that time, and in ancient times, without the master's order, the general was not qualified to go to war without asking indiscriminately. This is also the reason why Xia Houdun, Zhang Liao and others did not play.
Second, Cao Cao loves talents, and there is no retreat.
This reason is not impossible in the background at that time. At that time, in front of Dangyang Bridge, Liu Bei's famous soldier Zhao Yun went in and out of Changbanpo with Liu Bei's child Liu Chan, surrounded by millions of soldiers and killed countless Cao Cao generals. Of course, there are some exaggerated elements, but Zhao Yun was so unscrupulous and invincible at that time, and his own courage was one reason. Another important reason is that Cao Cao ordered not to sneak attack with arrows at that time. This is obviously Cao Cao's love talent.
In addition, Cao Cao knew that Liu was a strange man, and he was cornered, but he had many elite soldiers, and Zhang Fei and others might be used by himself in the future, so at Dangyang Bridge, they didn't want to kill Zhang Fei and Liu Bei who had not gone far. So, instead of letting Xu Zhu, Zhang Liao and others go out to attack, they ordered the withdrawal.
Third, the fire in the new field is a lesson from the past.
As we all know, before Cao Cao pursued and defeated Liu Bei, he sent Xia Houdun and others to attack the Xinye guarded by Liu Bei. But because of Zhuge Liang's clever calculation and Xia Houdun's reckless underestimation. The army suffered heavy casualties and was defeated.
This war made Zhuge Liang famous, and with the praise of his advisers, Cao Cao was afraid of Zhuge Liang and knew that Liu Bei had Zhuge Liang's help, so he should be careful in everything.
It is one thing to be afraid of Zhang Fei's bravery on Dangyang Bridge. He may also be afraid that this is an ambush by Zhuge Liang. After all, there is a bug in Zhuge Yan Yi. If we rush to kill the past, like the battle of Xinye, the soldiers will not be bloody, and the soldiers will not be bloody. So we have to retreat first and wait for the right time. Under such circumstances, Zhang Liao, Xu Zhu and others can only retreat, not to mention Xia Houdun's fiasco in the Battle of Xinye.