I won't claim to speak for gen Y / millenials, but for (pre) wartime babies, boomers and gen X, here are some of the reasons I have heard from friends and colleagues, or read about:
- people remember that British rule was not all wine and roses; restricted educational and job opportunites, toleration of bullying and other mistreatment of locals by the then overlords,
- they and their parents genuinely became better off under communism (which ironically allowed the kids to move to HK),
- the deep-seated cultural desire for stability and food security over freedom of speech (Maslow's heirachy, anyone?),
- the belief that in 100 years none of what is going on now politically is going to have made any difference; it's more important to care for the "inside little" people (family) than to worry about "outside" people and circumstances,
- racism,
- the idea that with China's rise in the world, for once they can be part of the winning team,
- $$$$$$$$$ (natch) and
- (not stated in so many words, but) little bullies like to side with big bullies so they are protected while bullying those smaller than themselves. Related to that, a number of triads have historically been allied with the CCP just as others have been with the KMT.