Why? Well, my scenario is this.
1. Soviet Union remains, ironically enough, isolationist because WW2 pans out differently, content to let the capitalists and Fascists beat each other up. The internal issues it has in OTL still remain however, they just aren't as... exposed.
2. Great Depression hits the US a LOT harder, as FDR... well, isn't alive. The US does eventually recover, but it takes much longer, and by the time it does, WW2 is wrapping up. Additionally, World War 2 is much more like World War 1 in this timeline, with all the issues that entails.
3. Germany is, "merely," Fascist, but also doesn't do as well against France. France and the UK get worn out fighting Germany, and Italy at the same time. They do win again, eventually, but it takes much longer.
4. China is able to beat off Japan, as war comes a year later, among other things. It still costs both immense resources however.
So, all the superpowers and potential ones are either exhausted fighting each other, or are too isolationist to care.
Point is, what would the world be like if there were no countries that could project power much beyond their immediate neighbors? Would it better off, or much worse off?