Military response would draw a lot of negative responses from the international community, and would be a clear rebuke of the "one country, two systems" that Hong Kong is supposed to be operating under. This could put HK's special economic status in jeopardy, which would be both damaging to Beijing as well as the HK business elite. It's better for the CCP if they can have the city suppress the protests using their own police force. After all, Western countries use the police to control protests all the time.