Horde healers really are fucking garbage, at least the vast majority of them. Alliance was clearly infinitely more organized. There's also the question of how shitty are Alliance healers relative to Horde? It's hard to make a comparison because, as I've said before, paladins are vastly superior pvp healers than shaman. And from my experience, Alliance healers are pretty bad, particularly the paladins because of how ridiculously low the skill floor is to still be effective. Chain heal won't save anyone from three mages AOEing. Shaman will go oom 5x faster than paladins, including mana tide totem (which you can just whack with any melee weapon). Paladins give every alliance player, regardless of whether or not they are in the same group and regardless of proximity to the paladin, a 10% increase to all stats. Throw in Titans which a lot of Alliance players were using, that's an easy 2000 health advantage.
It doesn't really matter how outnumbered the Alliance were. It's a LOT easier to organize a small number of large groups (guilds) than a large number of small groups (guilds), and that organization will easily overcome being outnumbered.
I'm just waiting for BC when shaman get revamped and are no longer complete garbage compared to paladins in every aspect besides raid healing minor aoe damage.