You’re partially right that this just hasn’t been explored in any great depth, though as you can see people have picked at the edges of it.
I think this is a pretty good resource for this kind of thing:
https://discourse.cataclysmdda.org/t/draft-of-the-cataclysm-lore-writeup/10345/3
SOME OF THE ABOVE IS WRONG. In particular a lot of stuff about societal change (super lax gun laws, drug use, widespread CBM use) was added to rationalize game elements, and is very subject to being reversed as some of those game elements are reigned in to be closer to modern day reality. For example the current direction for guns is to model present-day gun demographics, this provides a nearly inexhaustible supply of home defence and hunting firearms for easy scavenging, as well as another near-inexhaustible supply of military ordinance that can be scavenged with greater difficulty. No special plot points are needed to explain it.
As for what killed most of the first wave (a quarter of the population or so), it’s a mix of:
Extra dimensional horrors that evaporated after the first surge.
Armed forces targeting above extra dimensional horrors.
Rioting and panic.
Transient environmental conditions like poisonous gasses and acid rain.
Reprogrammed homicidal security robots.