First and most important reason why Hong Kong is not China: You have to pass immigration. You officialy leave China (that’s why we needed a double-entry visa) and get a Hong Kong stamp. The traffic is left-sided. The buses don’t look like they’re going to fall apart any minute, and their exhaust fumes don’t smell like they could kill you instantly. There’s no grey and thick smog covering the sun. The bus drivers are not criminals trying to kill a whole busload of passengers at the same time by crashing head-on into a truck. The streets are reasonably clean (well, then everybody is reminded at every street corner to “keep the city clean”). There’s no smell of rotting water and humidity everywhere. Kids don’t shit on the street. People are allowed to gather in big numbers in parks for picknicks, for example for the Lantern Festival. Everybody can express his or her opinion. Journalists are allowed to write critical articles in the newspapers (for example, about the 60th anniversary of the PRC). You can buy international newspapers and magazines. You can watch interesting news on TV and access every webpage you want without feeling like a criminal. There’s a reasonably transparent political system and a judiciary system that respects human rights. Hong Kong hospitals are among the best in the world and healthcare is accessible to everybody.
Hong Kong is not China, and will, in my view, probably never be. How could you replace a democratic system that respects human rights with what there is in China at the moment? Hong Kong would loose too much, economically and politically.