Blizzard is very political when it comes to changes. One of the reasons they have taken this position is flailing when changes happen to another class or spec. If your class/spec is getting nurfed than no one better be getting buffed. You can see it all over the wow forums. So how does this pertain to priest? Well I am not sure how it started but at some point Disc has become Blizzards baby. There has been more changes to disc than any other spec for 3 expansions now. Now they have a full on revamp in my opinion with changes to crit and aegis.
While I don't feel that Holy is broken they logs clearly show disc pulling higher numbers on almost every fight. Sure you can say "that is because all the good priests are disc right now, if they went holy than holy would be at the top." If that were true it would be a more of what spec do you like better kind of deal and there would be more even amounts. It could be that disc is just more fun to play but that would not matter to top guilds who would make players play what is best not what is fun. Politically would it be smart to buff holy when they just changed disc? Not really, there is already a ton of out crys to revert the disc changes. While I personally think the disc changes were a buff and a awesome dynamic to add to wow people fear change.
I love priest and I don't care to see any of the spec suffer. The truth is disc brings so much more to the table than Holy. Holy the original healer use to be the pillar of healing. Look at the icon on the healing forums on battle.net. It's a Holy priest! So why does the Holy priest parse so much lower on 90% of the fights vs disc and yet disc still pulls an amazing amount of damage? I can't answer that, only blizzard can but what I can say is it wouldn't be the best move to buff Holy while changing Disc. I see a great deal of politics there.
It saddens me to see Holy in the state that it is in always in the shadow of it's disc brother. My heart goes out and I will pray that balance will be found.
If I could name off one easy fix it would be Spirit and intellect. Disc can stack so much more int not needing as much spirit as Holy. If Holy was given better regen or a reduction in mana cost it would allow for more int stacking and would level out it's numbers with disc. Bam, easy fix. maybe in 5.3, lets hope Holy will one day return as it once was.