Because if the youth in Mainland China riots against the Party State, they would be "disappeared" into undisclosed jails, tortured, and given a permanent mark down on their social credit. They would then never be able to join the CCP or work for any government affiliated entities. They might even have problems borrowing money to start businesses or buy properties. In other words, if you dissent against the Party State, you can kiss good bye to a good life. Also, both Confucianism and CCP emphasize respect toward authority, while HKers were brought up under certain liberal norms, despite the fact that the Brits never gave them democracy.
And if the youth in USA or UK or anywhere else in this world riots against their goverment, they will be praised? And after rioting they will have a good life ?