My guess is that this is likely because of the availability of an always on display. Apple has likely programmed each of the watch faces into the microcontrolle

My guess is that this is likely because of the availability of an always on display. Apple has likely programmed each of the watch faces into the microcontroller that manages that low power mode. The other option would be to wake up the main cpu every time you need to refresh the 3rd party watch face, but that would have a power cost.