Why are some apps restricted by Region in Hong Kong?

This may be a global society but there are still individual countries all with their own licensing and content laws. Apple works on a country by country basis. Developers get to decide where they want to offer their apps (unless prohibited by a law of a country). The standard approach is to send the developer a request to broaden distribution of their app.

There is no real solution. For those who reside in multiple countries and absolutely must use apps from each country, a solution is to set up separate account on multiple devices, then use the device for the country's services to which you need access. Yes, that is costly, but so are a bunch of other things associated with living in different places. You can save a bit by using an iPhone as an iPod by only using wifi and no cellular service.