Write a question about the following article: Since 1444 Si Jifa had repeatedly sent tribute to Ming asking for pardon to no avail. In March 1449, a combined army of 150,000 soldiers was amassed, and the fourth and final campaign to extirpate the Mong Mao threat was launched under the supervision of Wang Ji. The army quickly marched on Mong Yang, which harbored Si Jifa, and captured their strongholds. However Si Jifa managed to escape yet again, and the campaign ended inconclusively with the ruling Shan elite allowed to remain in Mong Yang so long as they never returned to Mong Mao. Sources disagree as to how Si Jifa met his end. One source claims he died in combat in 1449, another says he was captured by the king of Ava and held captive in exchange for Ming territory, and one Shan chronicle claims he reigned for another fifty years. While Mong Mao had been defeated and pacified, Si Jifa's son Si Hongfa continued to rule in Mong Yang and his successors would eventually go on to invade Ava in 1527. So in practice Si Lunfa's family survived as rulers in the neighboring state of Mong Yang under the close observation of Ming. The fourth campaign was also marred by lack of discipline, inefficient administration, and mismanagement of resources. On the first day of mobilization, the entire 150,000 strong army started marching all at once, and many were trampled to death. Food supplies were mismanaged and assigned to individual carriers disproportionate to their weight, and no proper plan for their distribution existed. Some soldiers committed suicide due to the conditions prevalent in the army.

Question about the article:
Why did he sent tribute?