Why did humanity leave the world in Pokémon Pokopia’s story?

Humanity left because the environment became inhospitable in Pokémon Pokopia’s story.

The game’s backstory describes inexplicable environmental phenomena that gradually made the landscape unable to support ordinary life. Humans eventually departed for outer space, leaving the Pokémon behind in a world that also became difficult for them to inhabit.

Before leaving, humanity stored Pokémon in a massive computer system designed to continue operating after the departure. The system could release Pokémon when necessary, creating the conditions for Ditto and the other characters to survive and begin restoring habitats.

This plot point is sometimes simplified as a conventional apocalypse or a conflict between humans and Pokémon. Pokopia’s story instead emphasizes environmental collapse, abandonment, and a long-term preservation system. The exact phenomena are part of the game’s mystery, but the resulting inhospitable environment is clearly established.