The same reason young, social media savvy altrighters in the west dog-whistle. There's parallels in HK nativism behind these protests similar to your familiar altright talking points. A bunch of mainland immigrants "locusts" to HK to buy up real estate and perverting civilized colonial values. At the end of the day, it's a bunch of ignorant kids stewing over lost economic and social privileges and reminiscing about an era they never experienced and realities that never existed. They dress up the root of dissatisfaction as a noble pursuit for suffrage because they are naive students who got radicalized on semester of liberal studies informed by western textbooks without understanding greater economic / geopolitical ground truths.

40 years ago, while mainland was still agrarian, HK had an industrial base and competitive advantage that supported a working class. Then Detroit happened to HK, and the only socially and economically "valuable" people left are tycoons and bankers. Now you have a generation of kids trying to subsist in one of the most inequitable cities in the world, too stubborn to pursue all the opportunities afforded to them in exploding mainland sectors where their English proficiency and western cultural connections will give them huge advantages. Instead they live with their parents in tiny apartments with nothing better to do but protest destructively while utilizing their valuable English skills and cultural competencies to brigade western social media in a counterproductive effort to hold off the increasing irrelevance of their city state like living in a world class / Chinese tier1 city is a birthright.