The best of any sport are schooled and moulded in it from single digits. How the hell is someone like Dan Carter - an athlete by any definition of the word - going to be able to get a scholarship to a University in the USA when he is cultivating his skills at Rugby in New Zealand at the same time the likes of Tom Brady are honing their skills at American Football? Think about it realistically, give everyone the same chance from 11 years old and you would see a wider variety of ethnicities in the NFL.
Theres a reason its called American Football, no other fucking nation takes an interest in it.
You also have to look at other things. We have a feeder system for three basic sports. Football, basketball, and baseball. Plus other sports as well. Hell, over here field hockey is a girl's sport, but I believe in NZ and other places more men play it.
Then look at the scouting, and scholarships. I am not saying that it's not a business elsewhere but, you are looking at a multibillion business. Like soccer.
Thing is, NZ doesn't doesn't have the genetic diversity, and the pacific /polys/Micro-nesians are not as genetically diverse as the Africans...plus, they are from islands. Pacific Islands are/were isolated from large continental land masses, therefore separated from large population with varying genetics, and varying harsh environments to adapt.
Rugby is primarily played by former English colonies---or English colonies that have a lot of a ritual in them, meaning they still look up to England as Mommy, and keep a lot of similar things, such a slang and accents.
America is the British colony that had said EFF YOU and we will do out own thing, our own way. And we became who we are because of that.
Which is why I believe a lot of this rugby animosity comes from. A lot of the former colonies identities are strongly tied to British forms of sports, culture, entertainment. Which can be a a good or bad thing. Their culture has not fostered the cultivation of their own separate identity, which is what the US has done with football, baseball, basketball, lacrosse (American Indian sport)...