This is purely a legality issue. The Civil Aviation Directory (HKCAD) sets limits as to how many hours a crew member can be on duty. Even if the crew wants to go on (and trust me, sometimes they want to get home as much as the pax), it would mean a huge fine for the airline and possible disciplinary action to the crew member.
As to why they would even leave with the possibility of a divert/crew change is two-fold:
1) It could literally be a question of a few minutes and by flying faster, getting shortcuts, more favorable winds, they might be able to make up the time.
2) The delay to get a replacement crew to the departure airport, if even available, would be much longer than taking off and positioning a replacement crew to TPE etc.