I can't answer for
@vilani99's thoughts on this but the trend of left-wing American future histories started with
Christian Nation about a decade ago, which depicted a Dominionist takeover of the United States after which they've have sort of become a stock villain. IMO its easy to see why - they are mainstream enough to be a plausible threat without alienating too broad an audience. I have some issues with how often they come up to be honest (and
Christian Nation pretty much embodies a lot of those objections) but I think its fine with this timeline where developing an original doomsday cult that only exists in what is effectively the prologue would be a waste of resources.
And to be fair, we don't know exactly what happened in the run up to and aftermath of the Staffers Putsch, only a rough outline. I think its fair to say that probably American state power had been declining for some time and from the articles in chapter 2, it seems that things don't immediately collapse into total breakdown right away. I can imagine that there is a period of escalating conflict between more conventional political factions coupled with breakdown of state authority in face of the civil war and mounting climate crisis. Perhaps like the Taliban in the 1990s, this version of the Dominionists emerged as a response to state breakdown in the interior and as a way of bringing the warlords to heel. I might be reading too much into it but its also even possible that the Dominonists who acquired nuclear weapons in the late 2080s had more conventional ideas about how they should be used and it was the chaos caused by the Worm that allowed the apocalypist faction of the movement to seize control. (This isn't to say that the more conventional Domionists were good... just that it might have taken a bit of a shove to move them from North-Korea-with-a-cross to fullblown Doomsday cult.)
One other thing I was going to mention
@vilani99 - did you read
A Short History of the Future by Warren Wagar - the broad outline of this timeline feels like a spiritual successor/modern update to that work.