1 Controlling internet speech
Many people believe that China banned Twitter and other websites such as Facebook because it wants full control over the speech taking place on the internet inside china.
2 To prevent riots
Facebook was banned in China in 2009 to prevent riots after a violent riot took place in Xinjiang. Months later, Twitter was also banned. Some people believe that Twitter was banned for the same reason.
3 To prevent spreading of rumors
After Facebook was banned, the Chinese government said that such websites, referring to social networks, are used to spread rumors. Twitter could have been banned for the same reason. (See Why Facebook is banned in China?)
4 User data stored outside China
According to Chinese regulations, user data cannot be stored on servers outside China. Because many social networks store data in their own countries, the Chinese government banned them.
5 To protect local websites
According to a message by Chinese President Xi Jinping, China wants to give a chance to its talents to grow instead of being forced to accept foreign companies into its market.
6 To protect the economy
President Xi Jinping said that putting restrictions on foreign companies gives a better chance for the Chinese companies to grow and so they can benefit the economy.
7 The Arab spring raised concerns
Even though both Facebook and Twitter were banned before the Arab spring, such an event still raised concerns and made the Chinese government believe that the banning was a correct step. Some people believe this is one reason why the ban wasn't removed. (See Why some countries banned Qnet?)