I have noticed that in many guilds, and certainly in all the ones I've been a part of, the healer characters tend to be the best geared. Gear that could go to a caster DPS or a healer, often ends up going to a healer. I think there are a number of reasons why this happens.
Raiding healers tend to have very secure raid spots. Healers either succeed or they fail. You don't sit a healer for underperfoming. You just kick them. So while a caster DPS might be asked to sit out for a particular fight due to raid synergies or low DPS, and thus miss out on the opportunity to acquire loot (or DKP, the monetized form of loot), competent raid healers can usually raid as much as they want to. That means they have the chance to acquire loot or DKP in a higher proportion than DPS.
Because the healers earn more DKP, when an item drops which could be of use to a healer or a caster DPS, the healer is more likely to get the item, if they want it.
There are also many pieces of loot which are very obviously healer loot and healers can get them cheaply. No one but a holy paladin wants spellpower plate. The only competition for spellpower mail is elemental shamans, and they aren't exactly droves of those. Resto druids might have a slightly harder time as you might bring more than one to a raid and there's a little overlap with moonkins (but you'll rarely bring more than one moonkin to a raid). Healing priests probably get the least benefit from the "healer loot" phenomenon because there are a lot more classes competing for caster cloth.
Since healers get obvious healer loot cheaply, they have more DKP left to compete with caster DPS.
I suspect, but cannot prove, that healers probably also engage in more bidding collusion than DPS. Good raid healing teams are not competitive with each other in the same way that DPS are. I could see collusion arising out of that.
Why It's Bad
The raid group benefits more from very well geared DPS than from very well geared healers. While you can always make use of more DPS, in healing there is a definite sense of "enough". As long as healers can heal all the unavoidable incoming damage without running out of mana, the healers are geared enough. And if your DPS is good, healers need less mana because the fights are shorter.
On really challenging content, healers being able to heal through avoidable damage is not going to save your raid anyway. The way raids are tuned at the time of writing, if you can't avoid avoidable damage, you are probably going to wipe. On content which is not challenging, well, I don't know why even write about that. There's some cases where even if you overgear it you can still die of stupid. For example Mimiron's rockets will still one-shot you.
However, on really challenging content a marginal DPS advantage makes the difference. How much more DPS does it take to prevent a 1% wipe?
Da Voodoo Doctor is In (Advice 5c)
- Why Raiding Healers Tend to Get Geared First, and Why This a Bad Thing