Elizabeth being trapped in Evermore contains a bit of parental worries when you think about her parents. Unless she was orphaned and Raised by Grandparents, it means that her parents had taken her to visit her grandfather, as parents often do, only for the parents to never see their daughter or her grandfather for decades — if ever again.
Why is Omnitopia the only region of Evermore that doesn't have any human population? It was most likely a decision that was made by Carltron because he didn't want there to be a potential "fifth column" in Omnitopia.
To elaborate, Carltron wanted to keep Professor Ruffleberg isolated and on a short leash. Having other humans living in Omnitopia would've posed a significant danger to Carltron's plans as their free-will would've required them to be placed under constant surveillance and management just to keep them content and avoid provoking suspicion among them. Some of the potential problems that Carltron didn't want to deal with from having space colonists include...
Colonists leaving Omnitopia to explore Evermore and end up scaring the native populations of Prehistoria, Nobilia, and Gothica.
Colonists leaving Omnitopia to explore Evermore and discovering evidence of Carltron's presence and interference across the world.
Colonists interacting with Professor Ruffleberg, their "Leader", and discovering that he is merely a figurehead who is being held under duress.
Colonists becoming paranoid about being constantly watched and controlled by someone in Omnitopia.
Colonists finally realizing that Carltron was trying to take control of Evermore and taking up arms in open rebellion against him.
Colonists merely living their daily lives in Omnitopia, which inevitably include producing waste and even breaking things, would've made Carltron's resentment of being created just to serve as a Butler even more unsustainable, given the fact that he's also a germophobic.
