Interesting question - there are of course probably an endless number of reasons, but a couple could include:
- Liquidity: Some claim that given the financial challenges in the US, the relative lack of liquidity in US markets may actually be impacting stock value

- Exchange effects: Typically businesses prefer to list in markets close to their country of operation. One basic reason for proximity is to allow for access to local currency credit - in an environment where the US dollar is increasingly under pressure, other markets closer to the businesses operational centre are becoming increasingly attractive

- Liberalisation of the RMB: The Hang Seng recently debuted the worlds first Yuan denominated ETF (Exchange Traded Fund - a type of investment asset). This is the first step towards the internationalisation of the Yuan, and is another driver for a shift of focus back to the Heng Seng

-Government pressure: Though largely unspoken, one reason for the shift could be political. As China grows its global economic influence, it stands to reason that pressure on Chinese companies to list solely in China would increase

Hope this helps. A couple of my random thoughts

Great answer Patrick! There could be other reasons as well:

- Cost of listing in US exchanges: It is expensive to list on a stock exchange. Moreover the listed companies have to pay an annual surcharge. Some exchanges charge by the volume of trades. In short, the companies could've decided that the expenses for continuing the listing in US couldn't be justified from a business perspective

- Scrutiny: US exchanges have very stringent disclosure requirements and companies have to spend tons of money on accountants to prepare accounts adhering to those requirements. On the other hand, exchanges elsewhere are not so stringent and hence it's easier to release annual statements etc. there

Just my 2 cents!