Many of the mainland Chinese investors are buying houses in order to get their money out of the country. China has been in a bubble economy for awhile and many people are nervous about when it’s going to pop. As well, the domestic housing market is in real trouble due to overbuilding.
Purchasing expensive houses in good, stable neighbourhoods is generally a good safe investment. Obviously not decent investment is guaranteed, but good houses in good neighbourhoods in stable countries are a much better investment than keeping all of your money in mainland China.