Could it be that slaves to darkness are not the primary bad guys?
Think about it.
Aos was first released and khorne was the major focus, Archaon got some love with the realm gate wars.
Tzeentch got some love with warhammer quest, then it was the season of war stuff with nurgle.
Beasts, Skaven and now slaanesh have gotten their times in the sun, with games workshop actively removing chaos undivided in 40k, why have a undivided focus chaos army in aos as a primary narrative force?
All four gods have battletomes, archeaon can plug into any army, expand out the dark oath stuff in a couple of months and let the midget chaos warrior models fade away being replaced by, blood reavers, blood warriors, mighty skull crushers, kiaroc accolites, tzaangors, hellstriders, blightkings and pusgoyal blightlords