I personally think that it's classy for them change to the traditional Thai dress while on-board and change it back to a business-like dress when off the plane. It makes to it more unique and exclusive for Thai passengers. The Thai silk dress is not to be worn wherever they want anyway. It is taylored to each individual. Imagine wearing them on a shuttle bus or at their cafeteria. Probably not a very good scene if it got dirty.

On a domestic flight or flight shorter than 1:45 hour, they are not to wear the traditional dress. For a quick turnaround flight, they can stay in the traditional dress for the flight back and change it back to the business-like uniform when they leave the plane. If you notice closely, female flight attendant uniform on the right side of the aircraft is a little bit different from that of the left side. Purser and above as well as non-Thai flight attendants do not wear the traditional Thai dress, by the way.