I don't think it is specific to only US, in India i get on average 10/week, I never had a problem in Germany though.. it depends on regulations i guess
I can imagine plenty of valid reasons for not being able to ship a brand new product outside of the US. The author didn't promise anyone that at least 6% or more of the global population could immediately order this product or anything.
> In any case, most of HN probably lives in the US.
I agree with your main point, it's not up to us where someone sells their product, but I'm not at all sure that most HN users live in the US; I don't have any more evidence than you, I would just assume most are outside.
It is significantly easier to sell many kinds of products in the US compared to other countries with large consumer bases.
That isn't always true, but could be the reason.
The US has relatively few regulations compared to places like Europe. Few enough that California can introduce new ones that become defacto national regulations.