Well, I told you that monsters were trapped underground, correct? We were kept there by a barrier constructed by human mages after we lost a war against humankind a very long time ago. It wasn't so bad!! We acclimated as needed, until it was decided that to break the barrier, we would collect any human souls that wandered into our midst and use their power for that purpose.

[He deliberately leaves out information. While he hadn't been fully interested in the details due to his egotistical nature, Mettaton knew enough--and had enough respect--not to talk about that which he had no personal stake in.]

Frisk would have been the seventh soul he'd captured. It just turned out differently, as luck would have it!!