I am still not sure if I understand the story in Fallout 3.

So everything is the same as the Earth's real history up until the 1940's/1950's, but then in a retro imagined future there is a nuclear holocaust in 2077, and when we start playing Fallout 3 the year is 2277?

But why not just say instead of the nuclear holocaust in 2077, that it happened in the 1950's?

Because between the 1960's to 2077 what happened? Why did civilisation advance so differently? Why is it still retro?

It's like there is a 120-ish year gap where nothing much happened and it's an empty time period.

Or is it supposed to be that the whole history is the same as the real Earth's but that in the years before 2077 civilisation receded and tried to emulate earlier time period?

I'm confused.