First, Wu Sangui's heart is not firm enough.
As the most critical defense line in the northeast of Ming Dynasty, Wu Sangui was located in Shanhaiguan area. In this area, he could have relied on his tens of thousands of troops to forcibly curb an ambition of the Qing Dynasty to seize the Ming Dynasty. When the Ming Dynasty perished, Li Zicheng attacked Xian 'an, and the Qing Dynasty entered the mainland from the northeast, Wu Sangui did not actively defend the Qing army, but under the coercion and inducement of Dourgen, his heart began to become unstable, and he had deep doubts about a series of policies of the Ming Dynasty and Li Zicheng. This allowed Dourgen to occupy Shanhaiguan, let Wu Sangui surrender, and gave him a chance.
Second, Li Zicheng's harm to the Wu Sangui family made Wu Sangui resolutely surrender to Dourgen.
Secondly, when Wu Sangui wavered between the Qing Dynasty and Li Zicheng's Dashun Army, Li Zicheng did not immediately woo Wu Sangui, but moved all the important items in Wu Sangui's home. Including some important jewels of Wu Sangui's family, were all removed, Wu Sangui's parents and wife were killed, and his beloved concubine Chen Yuanyuan was robbed. This made Wu Sangui have a very disgusted attitude towards Li Zicheng, and at this time Dourgen took the initiative to surrender; Wu Sangui, with tens of thousands of troops, gradually surrendered to Dourgen. It can be said that Li Zicheng took the initiative to let Wu Sangui surrender to Dourgen.
To sum up, the reason why Wu Sangui, with tens of thousands of troops, voluntarily surrendered to Dourgen was that his heart was shaken in the late Ming and early Qing Dynasties; In addition, Li Zicheng's unfair treatment of Wu Sangui's family members and relatives, including finance.