Board Thread:Warhammer 40k General Discussion/@comment-94.3.72.36-20131224225315/@comment-18016240-20140227165725

Big E probably saw that with the Butchers' nails and general brutality of the planet they grew up in they were pretty much Khorn flakes from the start, he also found out that the Butchers' Nails were killing Angron slowly and that he likely wouldn't live to see the end of the Great Crusade, so the Emperor deemed Angron a lost cause and went back to his more favored sons (Horus, Rogal Dorn, Guiliman, etc).

We also have to take in the Emperor's personality, namely his flaws. In "The Last Church" (wonderful and insightful short story btw) we see that the Emperor is driven to the point of obsession when it comes to his beliefs, to the extent that he saw anything that didn't have to do with the Great Crusade or his Imperial Truth as irrelevant and either ignored or outright rejected them. He also didn't really see most of his sons as his sons, other than Horus he thought of the Primarchs, astartes, and humans in his service as tools, not people with thoughts, needs and desires of their own.