Wait! Screw that! I just remembered the lore.
The plane where cyrodril/morrowind/skyrim/etc situated was originally part of oblivion. That's why gates to oblivion can be open infinite times all over the places. Mehrunes Dagon invaded cyrodril because he wants to take back the plane. When Martin smashed the amulet and transformed himself into the dragon and killed Mehrune Dagon. This stopped any future gates to oblivion from opening in the land.
[ 08-14-2006, 07:35 PM: Message edited by: Kakero ]