I'm surprised it's banned there... how could Snapchat possibly be threatening to the Chinese government? Are selfy videos really danger to the party? Were the keyword filters not effective?
1) Ephemeral messaging. They want to keep records of what people are saying.
2) Reputation for sexting. Pornography is a major target for censorship in China.
Any messaging system that the Chinese government isn't able to assert control over and have access to gets banned.