In mainland, people sees the government as the ruler. They are not there to serve you, they are there to manage you. They never make mistake, and you must follow their lead.
The government needs Hongkong, and hey need Hongkong under control, just like the rest of China. If they failed, their public image of power will be damaged.
However, if they can turn Hongkong into "the rest of China" alike, then that's a success showcase of power for them.