I think the defacto 'Holy Radiance + Light of Dawn, Lol top meters' is a bit unfair.

When you consider all the fights in DS, think about it logically.

Morchok - Tank damage can be equal to melee damage, AoE hitting Melee + tank viable to tank heal
Zanozz - Group up phase all healers AoE heal.
Yorshaj - Many times where AoE healing when everyone is packed can keep tank up as well.
Hagara - Tank healer basically gets most of the healable damage, and if the tank is good he can assist even with raid damage
Ultraxion - Constant AoE, even the 'tank healer' can become an AoE machine. Later half of the fight, so much AoE healing is present tank-healing paladin can just AoE heal. (Hence pallies scream bloody murder if they don't get blue)
Blackhorn - Lot of times you stack up to heal, tank damage only picks up at end of fight.
Spine and Madness - Definately more raid-heal heavy.

As you can see, these days, just saying 'I'm a tank healer' isn't what it used to be in wrath. In wrath, you healed the tank and yourself and didn't stop. Now, you are expected to pitch in for AoE. That's why a lot of guilds are 2-healing most of DS, is because so many classes can actually succeed at both. The world of cut and dry tank/raid healing has been gone for a while now. Many fights leading up to now were the same, It's just that now paladins were given AoE tools to handle them bringing them up to that level.

Personally, I believe some of the gap between healers is still skill based. Paladin healers have always had to force themselves to be very mana aware ('infinite' is a generalization. That blue bar does move, and somtimes like a see-saw where missing one swing and you're done. It's a rhythm). They also are generally the largest subscriber to the ABC's of healing. (Always Be Casting) Alternately, shaman has been changed every patch or so, priests have two healing specs. It's no wonder that the two classes that have had the least impacting changes, therefore most practice at what they do, are towards the top.

tl:dr Holy Paladins have an easier time at the top because of few changes and a spell they can treat exactly like divine light, making life easy since they have not changed tons.