A lotta people forgot about it! But a looooong long long time ago, monsters were livin' on the surface with humans. But humans got scared, because monsters can take human souls but monster souls don't stay long enough for a human to take them. So the humans attacked the monsters and put them underground and cast a spell to put the Barrier there.

[There's more to the story, but Frisk never learned about the nitty gritty details. Just the broad, important strokes.]