Probably because of legal restrictions. If you have already taken off late from your origin destination, then, with the length of flights from a lot of the US destinations, an additional 3 hour delay before departure will take crew, both cabin and flight, close to their legal maximum flying. To then add on the extra 4 (?) hours to HKG would make them illegal to fly.

Crew tend to have no control over things like this, and as much as they probably would very much like to go home, if they become illegal to fly, then it is a done deal, they are not going home and you are having a crew change. It has happened to me several times in Europe, where I have already worked a 13.5 hour day, and end up having to come off a flight, or stay an extra night away from home because to fly the next sector, or operate a my intended flight the next day would not be allowed due to high hours, or not enough legal rest between flights.