Well, maybe because DDs have 0 control/info over when such affixes drop?
Like, when I am playing Retribution and I just casted my Avenging Crusade, building stacks and suddenly a Afflicted Shade/Spirit appears, that means I as a DD have to potentially ruin my entire high damage rotation just to help the healer get rid of that Affix-Threat during the fight.
I can totally see why DDs rather take the hit than block it when it comes to such affixes.
But if we as players would have a visual Affix Timer or something that announces the next incoming shade 1 Minute ahead, people have at least time to plan accordingly and fight that way.
In Raids and Dungeons we can read the Boss energy bar for the mechanic cast. But we can’t do the same for affixes.