The entire world was not nearly as homogenous in the 1950's. If things froze culturally in the 1950's for all countries as they did in America in the Fallout Universe - the style of the game would be completely different in another country. A 1950's Americana Vault Boy wouldn't make sense. Everything that is 'Fallout' is based on the notion and history of American companies, the American government, etc etc.
There would be new things to explore in other countries but it might as well be a new game. No Vault-Tec, No Enclave, No Vault Boy, No BOS, No FEV (I guess?), No Super Mutants. What would be the point of transplanting the brand. Might as well be Stalker except set after a nuclear war instead of Chernobyl.
Granted, other countries may not have the same 1950's feel to them, but if America stuck to it's 1950's view of the future, then maybe other countries stuck with their ealier views of the future as well. Maybe Germany clung to it's WWII third reich ideals, maybe China kept more of it traditional sesibilities rather than becoming communist, maybe Japan wasn't influenced by Hiroshima/Nagasaki to abstain from nuclear weaponry.
How do you know that Vault-Tech wasn't selling Vaults to other countries? If they were, then their "Made in America" pride would probably have made them keep the Vault Boy and any other company propaganda logos and such.
The BOS may not be the BOS, but to me seeing the Samurai of Steel in Japan, or the Iron Knights in England, or other such paralleles would be just as interesting. Or, if the BOS can use airships to cross the US to get to the east coast, then why couldn't they have also crossed the Bering Straight from Alaska into either Russian or the orient?
How do you know there's no Enclave? If the Enclave had their main base out in the ocean, and saw everyone that wasn't Enclave as being mutated scum that needs to be exterminated, then maybe they started attacking other countries too. Maybe raids on other countries is how they kept their production and scientific superiority. If the BOS in the Fallout 3 timeline has divided into two factions (west coast vs east coast), then maybe there's a breakaway Enclave group overseas as well.
As for FEV or Super Mutants, I already suggested alternatives to that.
Although I think the 1950's thing in Fallout is charming, to me it's not the end all be all of Fallout.