^I guess the writers figured an invasion force from the Unknown Regions wouldn't make for a credible threat considering how small the Unknown Regions are compared to the rest of the galaxy, so they went with extragalactic aliens instead.
Of course, the Unknown Regions should have been larger in the first place. The various writers of the modern EU fumbled the ball when they made the Republic/Empire encompass most of the galaxy instead of just a small portion of it.