So the Great Rift is also known as the Crimson Path. Abbadon's very plan to extend the Eye of Terror with a spear of unreality all the way to Terra. But something must have gone amiss as the aim was way off the mark. Perhaps the dark gods intervened and thought merely cleaving the Imperium in twain would be more lulzworthy and emotiongiving than going directly for Abby's endgame at Terra. Could be seen as a bit of a snub to him too, because he refuses to bend knees to them.