It all boils down to money and opportunity.
There are only so many large athletic men, even in a large country like the US. In the past boxing was a very good way to earn money compared to other sports like football and basketball, now the situation is flipped you can still make great money boxing but getting to that pot of gold is exceedingly difficult, while it is not easy to become a pro basketball or football player the number of guys who really make it every year in those sports is more that make it in a decade of boxing. Even if a guys doesn't make it in those sports the path to the pros leaves them educated if they take that seriously, where as if you don't make it in boxing you are essentially assed out.
Plus a team sport is not as hard because you have people to cover for you, in boxing it is all on that one person and that is very stressful although it might appeal to some people.
It is true that other countries deal with it also because there is basically an unlimited need for large athletic guys.
The positive side of it is in boxing all a country needs to do is come up with one or two guys and they can be dominating boxing for a decade~