Has it not got something to do with the pay and grading for locals.If you get Radar ratings then you become an Atco 2.There are a few 'why' questions in there so you won't get any answers from HKATC management.I would say yes you're right,but then I tried to get on APP for 2 years,and left because they just weren't interested.They also put some younger gweilos in the TWR,and older guys on APP radar.Figure it out?
Also you could argue that the TWR is the first stepping stone,to get experience etc.Not a bad thing.It also sounds like they have a training log-jam on the 6th floor.Can't comment any further,since I don't work there anymore(and don't care).However maybe someone from HKATC can comment,or are they too scared for their contracts.