A version of the "Why CONUS only?" thread exists on pretty much every watch forum I've ever been a member of.
It seems to be a primarily US thing with the fear of shipping to anywhere outside the US. The idea that someone from the US would have the "handhold" someone in another country as some have said seems odd to me. In most other countries, shipping internationally isn't unusual, so in my experience at least, people know what to expect, so I think this more of a thing from the sender.
Those who talk about the US market's size are right - many watch people in the US have never shipped internationally, and haven't had the need to, so for many it's an unknown they just don't want to deal with. I've had people in the US send me watches to service who had never shipped anything internationally before, and sometimes when they show up at their post office, the people there have no clue how to do it...amazing.
Having shipped internationally for years, there's really nothing that's a big deal about it, or a big risk. This US only exclusion doesn't seem to be common here, but there is a seller I see who puts this in their sales threads pretty often. There was a watch I was interested in, but just like US sellers don't want to be bothered with the "hassle" of international shipping, I don't want to be bothered trying to convince someone to sell to me, so their loss.