This is because Blizzard wanted to make the insentive that healers should be able to level on their own merits. As in, healers should be able to quest without having to go DPS or Tank.
As such, they buffed their damage. At higher level this is unnoticeable but at lower level the DPS specs do not have all their spells, their interractions between spells, their intricacies that make the spec more complete. Healers however already have all their spells. This means that at lower level a healer does more damage than most DPS specs as a result.
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this is wrong imo
it doesn't really have anything to do with ability combos. boomkins pretty much have their core combo at 19, arc mages just use 1 ability, and they do comparably strong damage. rshams do insane dmg with flame shock, mw monks with teachings of the monastery passive (which is an ability combo.)
this argument just doesnt consistently apply. also there's the basic logic that any damage boost to help healers level should NOT result in MORE damage than a pure dps spec. mw monk does astronomically more damage AND healing than ww monk which makes no sense, but they do less damage than arcane mage (a one-button dps spec) which makes even less sense.
if it's blizzard's goal to make healers more viable for leveling, then it doesn't make sense to make dps specs within the same class LESS viable for leveling. the penalty of living longer means killing things slower, that's how the game has been since day 1.
TLDR it's just a scaling problem and blizzard has sucked at balancing anything for 14 years.
as leek said healers should just get a massive damage nerf, but blizz will not balance the game based on lower brackets. thats why arc mages got buffed while enhance shamans got nerfed.
also warriors do good damage but are not a good class in this meta just throwin that out there Bv)