It was because a group of rebels formed. It was led by heterodox Christian convert Hong Xiuquan, who, having claimed to have received visions, maintained that he was the younger brother of Jesus Christ. He led an army that controlled much of southern China, with about 30 million people. The rebel Kingdom announced social reforms and the replacement of Confucianism, Buddhism and Chinese folk religion by a form of Christianity, holding that Hong Xiuquan was the younger brother of Jesus. The Taiping areas were besieged by Qing, the normal government, forces throughout most of the rebellion. The Qing government defeated the rebellion with the eventual aid of French and British forces.

.. So.. I guess it's since they sided with that rebel..?