An under trained, sleep deprived, drunk, or high Thai driver is not an appealing prospect for anyone coming to Thailand, everyone knows how bad Thai drivers are and how dangerous the roads are because of their zero competence behind the wheel.
The chins are no doubt aware of this and choose to put one of their countrymen in the drivers seat.
If i had the option of a western driver to take me somewhere rather than a Thai, i would jump at it. Being in a vehicle with a Thai driving terrifies me.