I dunno, I personally feel Americana is an important element of the Fallout franchise. All-American 1950s family values and Cold War culture are vital parts of the games' propaganda and aesthetic.
I suppose Russia could work, because Russia is a country that seems like an appropriate setting for post-apocalypia. It's a vast landscape that lends itself well to bleak settings, and even today there are abandoned bunkers, silos, bases, and other remnants of the old Soviet military-industrial complex littering the landscape. Not to mention the fact that Russia was a major player in the Cold War.
Besides that, though, China is simply too alien to the setting, and smaller European countries just don't have any emotional resonance for most of the target audience.
I agree with most of that, although China would be interesting precisely because it's the opposite side of the great war. I'd like to see propaganda posters of countless brave chinese soldiers overwhelming capitalist devils in their nefarious armor suits.
Russia is kind of bland in terms of geography because of its size and general uniformity. It's basically just a big forest and some tundra. China has vast mountain ranges, tropical jungles, parched deserts, the great wall, great cities with a mix of modern and ancient architecture, etc. etc. etc.
And in case you didn't notice, Fallout's not set in the real world today, so it doesn't matter how many abandoned bunkers or bases Russia has, fictional China probably has more considering Russia is mentioned in Fallout once and Fallout 3 once and never again. It's clear that Russia is not important by the time the Great War occurs in Fallout's timeline.