Why, you ask?
Kind of like that time they removed the plagued and black protodrakes, from the meta achievements of the first raid dungeon in the Wrath expansion, they think the rewards will be too easy to get once the expansion is out. Which, easier to get, yes they would be. TOO easy is debatable since the dungeons reduce your item level upon entering, but, sure, whatever you say, Blizzard.
Now, one thing I like to do is "retro" raiding, to go back to those old dungeons, and get the cool stuff we just weren't quite "leet" enough to get when it was current content.
Basically by removing things like the plagued and black protodrakes, and now these dungeon mode rewards, Blizzard is catering to the "elite" and "hardcore" group of players. The group of immature whiners that get upset that they aren't "special" anymore for having done things when they were relevant. Give them some damn feat of strength achievements, then. But otherwise, fuck those kids, I pay for this game, and I want access to all of that crap, too! Getting it later is fine with me, not having a feat of strength achievement is fine, too. But I'm an adult and I don't have the time to sit here, hours on end, mastering the dungeon in order to get the stuff when it's relevant. By the way "relevant" means "current content" which also translates too "takes for fucking ever". How long is the Garrosh fight at the end of the Siege of Orgrimmar raid? Seriously, like 15 minutes? Our group was dying to that fight in the last phase, so that's toward the end of those 15 minutes, which means wiping to Garrosh twice takes about half an hour, and that my friends, is a waste of my precious free time. And when you're an adult, that free time is Precious.
The trouble with the "cater to the whiny little 'leet' kids" strategy is that this is not what they usually do....(which is good because if they removed every thing like that after it's not "relevant" content anymore, I would have quit playing WoW a long, long time ago).
For example, just, a couple months ago I got the Glory of the Cataclysm Raider. The reward for this meta achievement being the drake of the East wind. There are NUMEROUS other such meta achievements with mount rewards, most of them some sort of dragon, it seems. So why did they decide to remove the reward from the very first set of raid meta achievements, because they'd be "too easy to get", and then proceed to allow ALL the rest of them to stay in the game, obtainable at any time you're bored and have some spare time?
I have no idea what the logic behind that is, actually.
Personally, I'm glad thus far that they don't normally remove these things, because as I mentioned, I PAY a monthly fee for this game, and if I'm not only paying for the game itself, but the subscription fee too, every, damn, month, I want access to ALL of the content. I don't want to pay the same amount to play the game as everyone else does, and then NEVER be able to have all the same things the kids that play twenty hours a day get to have, because I am an adult with a JOB, and other things to do, like spending time with my boyfriend, taking care of my dogs, eating, showering, cleaning my house, fulfilling family and social obligations, etc.
I really, really wanted that damn plagued protodrake but the leaders of the group I was raiding with at the time didn't push people to do the achievements at all. I don't even think they tried most of them until the mount was already unavailable.
So, I'm now killing myself, playing a lot more than I want too, doing things I don't really enjoy doing (farming) trying to work up enough gold to buy a carry through the challenge modes in order to get the mount and the gear for my warlock. I will have NO gold saved up for the upcoming expansion, because if I don't do this NOW, and spend every single copper all my characters have put together, to get just one of them through a gold challenge mode run, I won't be able to get the Pandaren phoenix mounts or the challenge mode gear. Which sucks, because I really like that mount, and I really like the warlock gear set, although I hate the hunter gear set, which is normal, I hate most of the hunter gear in the game. I think I will only be able to get ONE of the mounts too, when I wish I could get them all.