several procedures and aircraft limitations here.
Restricted Max speed with all gear and doors not properly retracted and stowed would not allow the aircraft to continue to destination with current fuel load.
Fuel dump can take up to 25-30 min depending on the fuel load and operation of all fuel dump pumps and valves. As well, as local laws. Fuel dumping can be restricted to certain areas and altitudes, as DAL found out when they dumped prematurely over LAX.
Abnormal Procedures can take 15-30 min to coordinate with company maintenance and dispatch personnel to make decision. Because it wasn't a "get it on the ground now" situation, they had time coordinate a plan.
BBK is the better choice for a gear issue. Longer runways (HKT one runway and would close airport) . BKK has more medical and emergency response capabilities. Plus the trip to BKK would burn another 20k-25k lbs fuel due to low altitude and restricted speed.
Unfortunately, they had an abnormal situation that could have easily progressed into an emergency. However, they had plenty of time to deal with all the issues. Crew ( flight and cabin), company dispatch, maintenance, air traffic control, airport authority and other aircraft in the area all need to coordinate a plan. Fortunately, they have several hours to do this.
Bottom line...everyone was safe. The aircraft manufacturer and airline have spent years to develop response to abnormal procedure and emergency procedure, procedures to most every conceivable situation.