Because right now it appears to not be suited for the spec so much (and Holy AOE mode appears powerful in it). There is very irregular damage unless the tactics are followed perfectly. I'm talking about a regular 5man that will do it clumsily. Forget for a moment that they don't do it properly and think about the damage an irregular fight in it produces. There is some tank damage which is relatively irregular since you don't know how he will pull. There is a boss ability that usually is not interrupted and does heavy damage (on 1 person?).
The result is that even if you spam 5-6 Prayers of Healing in the beginning there is still heavy damage. I usually end up with the 5man winning it but the health of everyone is very low or 1 dead.
(the ilevel is around the 5man, not overgeared for it)
Prayer of Mending doesn't appear to do that much either due to the heavy damage. Even if I resort to Greater Heal individually, it doesn't appear very efficient.
tl;dr: if that fight is done clumsily in a regular random 5man, the raid damage is very irregular and shielding can not be done reliably(since people get random heavy damages)/ or other healing.
I tend to think the Disc Priest does not like irregular damage that is done in high numbers and at random people (that can't predict who it will be).