The same reasons why some countries banned WeChat, the most popular Facebook-alike app in China.
TO Sum Up:
1. FB did not give China access to the users' information. Also, the gov believes that Facebook could be a platform to be used to spread rumors that might threaten the safety and censorship, and it would not take that risk.
2. To take the share of Internet business and push the economy forward. With the subsequent rise of Chinese digital giants like Alibaba and Tencent, you might argue that this gesture could be one example of economic protectionism.
3. Better control over internet content inside China.
4. Political problems, riots and government. After a series of riots in XInjiang, China blocked Facebook.