Idk bout Shadow but Aff’s DoT % is around 70% of its damage from DoT damage. It just takes all 9 DoTs totaled up to get there, which is absurdly dumb, annoying and irritating.
If you mean why does Moonfire and Sunfire do more damage % than Corruption, Agony, UA etc. then that’s because DoT damage is less per DoT itself since there’s 7-9 DoTs total on the spec.
As for Shadow, no clue. I’d suspect it’s because between the spirit mechanic and people who keep pushing DoT specs to be less about DoTs and more about other things, that’s probably it, there.
Also I don’t see anywhere where any boomkin’s total DoT damage is higher than Affliction’s.
As far as Shadow goes, adding up the total % of DoT damage n whatnot without including tier bonuses, Shadow’s looks to be dead even at around 50/50 on multi, on ST it’s DoT damage is quite terrible because they just don’t do much damage at all.
Blizzard, for this and last expansion, kept trying to get DoT specs to be less about DoT damage and more about maintaining DoTs while doing other things, which is absurdly dumb but oh well.