Because before western side of Dwilight had a lot of monsters before and that is why they chose to do it again. But its not fair to any western realm that have worked hard to establish themselves. They had to do something but destroying one whole side is not the way to go in my opnion. Destroying regions on the edges of the world evenly across all continents would be a lot better choice instead of just !@#$ting on one side..
