>>57279700there's no real explanation, but it's a reasonable guess that humans are not native to the PMD world. if they were, there'd be an actual population instead of a few scattered instances across however many years, so it's likely they just get isekai'd there to help the inhabitants who seem VERY fond of doing that whenever there's a crisis. there's also the fact that everyone seems to know what they are, but basically no one has ever seen one, which hints to them being almost mythological in that world. everyone in our world knows what a dragon is but obviously nobody has ever seen one.
anyway, in gates hundreds if not thousands of humans are summoned by hydreigon and all of them failed. we don't get a concrete number but it's implied that kyurem has been constantly hunting them down for a very long time. thanks to gates we also know that it's possible to travel between the worlds freely, so maybe some more scientifically-minded humans who failed and got sent back to their world tried to find a way back to the PMD world and succeeded. maybe they also built the lab and power plant. pokemon is no stranger to interdimensional travel, after all.
as for why it's abandoned now, who knows? could be any number of things.
