Why did the pre-Qin academic lose?

Hundred schools of thought in the pre-Qin period was not completely lost, but declined. Qin Shihuang burned books to bury Confucianism, and Emperor Wudi ousted hundreds of schools. In order to survive, various theories were either reduced to cheap learning and spread among the people, such as Mohism; Otherwise, it will be integrated into the Confucian system, which is called a part of Confucianism, such as Legalism!