It probably helped that one was at least nominally socialist, so the promise of worker liberation was always on the wind, keeping the capitalists a bit nervous like are they serious? what if it actually happens?

I don't much like hypotheses that require certain size groups of people.

But I guess that's just the instability of the dictator applied to the entire world.

If you crack open a history book you'll find that a multi-polar world has a lot more war. Napoleonic wars, World Wars, etc. Even in the Cold War when nuclear weapons made it too insane for super powers to fight each other directly, they found ways to have massive proxy wars, like the Korean War and Vietnam War. These wars left over a million people dead each. Then there were all of the various civil wars that the superpowers were involved like the Angolan civil war, that one was about a half million people dead. These conflicts were happening all over the world.

People today shit themselves over Gaza, but that's less than a tenth of casualties compared to the wars that happened in the Cold War and less than 0.2% of WW2. Would you consider wars happening in multiple parts of the world with 10 to 500 times the casualties we've seen in Gaza to be a "stable world"?