Why did Sun Yat-sen resign as provisional president in 1912?

Sun Yat-sen resigned as provisional president in February 1912 primarily to facilitate the unification of northern and southern China and to ensure a peaceful transition of power following the collapse of the Qing dynasty.123

At the time, Sun recognized that his revolutionary regime in Nanjing was relatively weak. Meanwhile, Yuan Shikai—who commanded the powerful Beiyang Army—had entered into negotiations with the revolutionaries after the Qing court proved unable to suppress the uprising.412

To secure the abdication of the Qing emperor and prevent further conflict, a deal was struck between the two sides. Sun agreed to step down so that Yuan could be chosen as his successor, thereby bringing the military and political forces of the north under the new republican framework. Following Sun's resignation, Yuan was elected president, and the government was eventually transferred to Beijing.24