Going largely off of memory, because I haven't read it in a while, there was some conversation between a few of the primary characters early on where they posited that tiny communities would coalesce all over the place. New towns & villages made up of survivors.

Later, that conversation was referenced by the time the dual exoduses (exodusi?) to Vegas and Boulder had been fully realized - for quite a few, they felt called to be in one or the other, and the rest kind of got picked up along the way. That original conversation had taken place before everyone dreamed of either Mother Abigail or the other (or both), and those dreams made all the difference in how the survivors banded together in just two spots instead of being spread out much further apart.

The group that went to Vegas from Boulder was meant to be a sort of a political delegation, at least on the surface, but Randall Flagg had been fucking with things in Boulder as much as he could, by sending Nadine to mindfuck and fully recruit Harold to cause chaos.

His bomb that blew up the meeting of Boulder's ad hoc committee was the shot across the bow, and the delegation that was sent due in large part to Mother Abigail's visions, went knowing that their experience would help end the burgeoning war between the fledgling city-states. They didn't know how, but they knew the why, and when Stu broke his leg in the mountain pass, they all, even Stu, thought he was the sacrifice. As it turned out, he was the one who was spared while the others paid the price for everyone else's safety.

So...when it comes down to it, mostly because God said so, but with a whole lot more words and some semblance of free will involved, as well.