I suspect it's simply because of the way the TH maps are delivered by the Asian tool 'Garmin Updater'. This piece of 'helper' software is not like GarminExpress although it does handle both Software and Map updates via 'launcher.exe' and will detect and update authorised devices connected in MSM. Point 1 of the instructions in TH & EN is "Please remove SD card from GPS device before updating map" so clearly it doesn't optionally update to a card the way GE can.
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I guess maybe it can be fooled into downloading to a card with GarminDevice.xml in Garmin folder but i haven't got a device suitable to try that fake nuvi/stunt double trick. Maybe someone in TH can test.