It might seem strange, given the system's surveillance and security networks, but the Communist Party is wary of high-profile law enforcement campaigns. Failure would be worse than embarrassing for a party which is supposed to represent the people's will. Such campaigns are only ever risked at times and in ways which demonstrate the Party's continuing hold on power.
That is from Bill Hayton's new book — Vietnam: Rising Dragon — which I found informative and insightful on virtually every page. Recommended.