the success of the superweapon
Super-weapons have been created many times. Chemical warfare agents were super-weapons: the press of the time hailed them as such, and claimed that they were so destructive, that war itself would become obsolete. The machine gun elicited the same claims. But none of these super-weapons worked. There wars raged on, and just became more brutal.
To answer why, we have to look at the super-weapon that worked. The super-weapon that ended wars between nations. In the nuclear age, wars are fought between small adversaries, with the major powers gaming the outcome, but political considerations outweigh the will to fight these wars.
Nuclear weapons prevent wars. Or at least they have helped, so far.
and the reason? Because the super-weapons that failed killed soldiers, and sometimes civilians. But nuclear weapons kill politicians. It really is as simple as that.