I found this interesting article that clears up the reasoning behind why the equestrian sports are being hosted in Hong Kong and not Beijing.
Why Hong Kong instead of Beijing?
February 22th 2008
In order for a horse participating in an international sporting event to be imported in a country, bilateral agreements – or protocols for temporary importation - must exist between the host country and the horse’s home country. This is needed to ensure that the equine health status of both countries is preserved or, in other words, that no infectious of contagious disease will be spread as a result of the trip.
So far, the equine health status of mainland China remains unknown; it is not clear what risks may be threatening international horses’ health. As a result, a horse that is imported in China will not be allowed back into its home country.
When the Games were allocated to Beijing back in 2001, it became evident that the temporary importation of 200 competition horses into an area with an unknown health status would be a serious challenge.
The FEI went to great lengths in trying to set up a Disease Free Zone (see below) in the vicinity of Beijing. However, due to a variety of factors and for reasons unrelated to the FEI, it became clear that this was an unfeasible objective.
It was then decided to move the equestrian events of the Olympic Games to the Special Administrative Region of Hong Kong, where protocols for temporary importation had been in existence for many years.
What is a Disease Free Zone?
A disease-free zone is a specific area which has been internationally recognised as being free of contagious diseases. For a zone to be pronounced disease-free, it has to correspond to some strict criteria. The number of equids in the area has to be studied; strict animal movement control must be set; and scientific (serological) surveys must be carried out to establish the exact disease situation. Furthermore, all the exporting countries need to agree that the survey and report are scientifically sound. It has therefore to be corroborated by an independent organisation, such as the World Organisation for Animal Health (Office International des Epizooties OIE).
Who Decided?
The decision to move the equestrian events from Beijing to Hong Kong was made between the International Olympic Committee (IOC) and the Beijing Organising Committee for the Olympic Games (BOCOG). Hong Kong was chosen in relation to its existing importation protocols and long-established equine health status. Another factor included Hong Kong’s expertise in dealing with sports horses available from within the Hong Kong Jockey Club, one of the oldest institutions in Hong Kong, founded in 1884 to promote horse racing.
And while being removed from the rest of the Olympic events has its drawbacks, it is widely viewed that the advantages in this case outweigh the disadvantages.
Hong Kong has been extremely supportive and the cooperation with the Hong Kong organisers has been tremendous.
FEI