Different countries have different providers, use different radio frequencies, etc and thus have different hardware and so on. Phones made for a particular market might not function well (or at all) in another market. This has historically been a barrier to cross-border adoption, though it has gotten a bit better in recent years.

Different radio channels are often used in different regions to get the bits from phone to tower and back. Software-defined radio only goes so far, at some point the hardware RF filters and the like have to be compatible.

Sorry, not sure what you're trying to say. I tried saying "a fairphone from the US would be more accessible than from EU because of shipping. I live closer to the US".

we use different frequency ranges for our cellular networks than most regions