Because that's an overdetermined explanation. Their primary motivation is control of information, and that by itself is the only reason they need to ban Facebook. It's also why they ban a number of other sites that don't compete with Chinese companies.

Protecting Chinese social media companies from competition is at best a secondary motivation and it probably isn't even necessary -- Chinese internet users prefer Chinese social media for a variety of reasons, and I don't think Facebook is a significant competitive threat. Neither is Twitter.

On the other hand, China allowed Google to operate even though it might have been a strong competitor to Baidu. Google was only blocked when it refused to comply with censorship policies -- again demonstrating that control over information is the government's main concern.