Mainly because of the last empeor, Zhou. His name was Xin in fact. Zhou, 纣 in Chinese is the name given by later generations meaning cruelty. It is the synonym of tyrant. He built a pool filled with wine and forest hanging with cooked meat. They coulc play and eat at any time. He could kill any one he want, for example when he want to know how a old man could cross the river in chilly winter without wearing a shoe, he cut off his feet to research the marrow. His people could not stand all his cruelty. The chief of Zhou (another zhou written as 周) rebelled and got the crowne.

thank you very much,but can you tell more about how did shang fall.

Hi, John! As you can see from Mr Qi's reply, the cruelty of the last shang emperor made his people living a miserable life. As an old Chinese saying, 'Losing the support of citizens, the emperor would definitely lose the throne.' The fallen down is inevitable.

The Shang Dynasty fell around 1046 BCE because of a very oppressive government.

actually there are more than just that. Yes that is one but not the main one. it fell because of how harsh the social classes were. people thought it was unfair.