Devices may be region specific for many different reasons.
The first, as we’ve already mentioned in this thread, is that the Z Wave frequency varies from region to region. This is because zwave operates in a frequency band which is often used by local first responders like firefighters and ambulance drivers. So the exact frequency sometimes has to be different in order to avoid interference with these safety services. The frequency cannot be changed after the time of manufacture, and must match the frequency of the hub exactly.
The second is that the electrical system are different. A smart plug-in device designed for Germany will not work in the US and vice versa.
The third is that some smart devices have encryption technology which is not allowed to be exported.
The fourth is that safety regulations can vary between countries. This may mean that a device which is legal to sell, say, in the US is not legal to operate in the EU. (This applies to a number of Zigbee devices because the US allows transmission at higher levels than the EU does.)
And fifth, quite a few companies do not want their products sold outside of specific regions because they do not want to have to meet the customer support and privacy requirements of a different region.
You put all five of those together, and it just becomes too expensive for them to have many different versions of the same device, and many manufacturers will choose to only sell their devices into one or two regions. It just becomes too expensive for them to have many different versions of the same device.
So device selection does vary by region. Some devices are available only for the EU, while others are available only for the US. And vice versa. As far as other regions like mainland China and Hong Kong, again, there’s just a lot of variation as far as what is available.