Why did Water/Horror Smoothie and the other Horror AU Humans leave the underground and never return? Did something happen to them to stop them from returning or resetting
Water never came back because that timeline was reset!
A chunk of my guys are simply 'game mechanics'. The True Neutral Path is the one you have to play first in Base Undertale yeah? Neutral in that Asgore dies because Flowey and he becomes Omega Flowey. Then Flowey goes 'wait we can do better reload/reset'. And you do, to do the Lab Date and get the True Pacifist Ending with fighting Asriel and all.
I think Horrortale is a biiiit different in that maybe some died besides Asgore but whatever mine is just, the timeline kept going on while the Reload/Reset one is the one Macaw and Lupo are in. It split the timelines.
So Water/Smoothie CAN'T go back. Doomed from the start sort of deal. Going back just creates new Timelines.
Technically it can be considered a 'dead' timeline, the story has stopped evolving. But Nightmare keeps it from Error and hangs around to farm tasty negativity.
.... Not that Vulture or Hyena care anymore, to be honest. They're more than happy working with octoboss.