They lost Hong Kong to colonial domination. The Chinese government legitimizes itself as the government that made China into a powerful state that could defend itself and act on its own terms after a hundred years of skyrocketing poverty, genocidal invasions, and devastating civil war. Which is true. China from 1850 to 1950 was 100 times worse than China from 1950 today, without even downplaying any of the awful things that happened since 1950.

Even if Hong Kong losing autonomy damages it economically, which I can't see happening to the extent that it wouldn't still make far more money than it costs, China controlling Hong Kong symbolizes the bad old days of the Opium Wars and Japanese genocides and warlordism being decisively over, not in the minds of China's rulers, but in the minds of the actual everyday people whose tacit support they need to govern.