What is the reason for the change of Han Dynasty's policy towards Xiongnu?

The reason for the change is the enhancement of its own strength.

At first, due to years of war, the country was initially determined and the economy was ruined. The Han dynasty adopted the policy of being close to the people and tried to keep peace with the Xiongnu temporarily. During the Jingdi period, on the one hand, the Han Dynasty continued to kiss each other, on the other hand, it also made resettlement at the border, imposed heavy orders and increased horses at home, strengthened the training of foot soldiers, and made a large number of weapons, all in preparation for resistance. During the reign of Emperor Wu of the Han Dynasty, the national strength of the Han Dynasty developed greatly, reaching its peak both economically and militarily, so the generals headed by Wei Qing and Huo Qubing launched three large-scale wars against the Huns. The Han Dynasty recovered the Hetao and occupied the Hexi Corridor, which cut off the connection between Xiongnu and Xiqiang and opened the passage between the Han Dynasty and the Western Regions, while Xiongnu moved awkwardly to Mobei, and the Xiongnu empire gradually declined from then on, and the Xiongnu threat was basically lifted.