History has shown that having a multitude of roughly-equal competing powers results in more per-capita death from war than when there is 1 or two dominant nations. The 1800's and early 1900's were bloody. Post WWII has had less death from war.
Bilateral is stable. Multilateral is not. We have limited evidence nukes change that. Currently, with the implosion of Pax Americana, we’re hearing words a multi-lateral theatre in the Middle East and Asia. Hence the heightened risk of nuclear power in one of those settings. (Europe remains a bilateral theatre.)
Maybe because of nukes now.
Or mass media.
True, though post-WWII was not a single power either until the 90s. We've had several decades of Cold War in which there were at least two great powers.