KapLt
Why- I am no expert. But IMHO.
All simulators (ATR, 330, 777) and until a couple of years ago (320) are located outside Vietnam, so it would increase the cost and logistics of screening.
As it is the local 320 sim whilst quite new, seems to have reliability issues, so I think even some 320 screening is ending up outside of Vietnam.. That might be the reason, then again I could be completely off base, might be some CAAV issue..