In japan’s case
A and B: Geography and culture. People tend to forget that Japan is the size of a large state. all of those players live in relatively close to each other when you compare them to the east and west coast in the US even east coast and midwest.
Its the exact same reason Cali has more high level players than montana, or New York has more high level players than Alaska. You get good by testing yourself against good competition and when good comp becomes something you have to travel for and not something you can get 10 minutes away from home. you have to start factoring in other things into the equation: travel time, gas money etc.
American players could rise to that skill level with noproblem if that level of skill existed in a place where they could see it and play against it everyday, but for most of the country that isn’t and is never going to be the case.
C) Oddly enough because two gaps exist If
(Ameican average)----------------(American tournament)–(Japanese Average)----------------(Japanese tournament)
Shifting the skill of of american tourney players closer to that of japanese tournament players would widen the gap between american casual players and american tournament players, mostly likely killing or at least severely damaging the american scene.
Also, for people who can’t read good, the question is about players in Asia and not why asian americans do better.