The way I understand it, it came to be like this because the outer boundaries of the map are rendered with much less detail, while to focus of the map still lies on the highly detailed Vegas/NTTR-region. The expanded map surface just won´t have sharp EDGEs where you abruptly "fall off" the the map (similar to the area around Tblisi on the current map, for example).
Perhaps this means that we will have elevation data for larger parts of the Grand Canyon to fly through, but it won´t be as pretty (initially). Who knows how EDGE will work with regard to the merging of adjoining, pre-existing map areas in the future...