This is going to get way out of hand.
I can give you very good reasons.
1) Canon wise America no longer had military bases around the world. America became extreme isolationists. The European Union fell apart, the United Nations fell apart. The world ran out of resources. America no longer had allies to protect or natural resources to protect. America was also fighting three wars at the sametime so every avalable military personal would be dedicated to them. The first War is the Sino-American War and second and third are the taking over of Canada and Mexico. America told the world to screw off and the world said the same back to America.
2) America would not have been selling or giving tech like Power Armour to other nations. Power Armour, Robots, Vault (as we know them), Laser weapons were all American only.
3) Other countries besides Canada and Mexico wouldn't have the American 1950s feel or culture. Again they wouldn't have the tech as well.
4) There is already so much of America left to see.
5) Going to another none English speaking country and have all the people speaking english would be stupid
6) No American factions would be seen or mentioned in a game outside North America. If they are it would be seriously stupid.
7) There would be no super mutants, and depending where the game is say Australia, there would also be no Deathclaws or mole rats.
8) Bethesda has already said they would not take Fallout outside of America. Keep in mind Canada was apart of America so maybe Canada in a future game.