Because it's freshest in everyone's mind, while being easily compared to the excellence of MoP. It's actually kind of interesting how it's worked out:
During TBC: 'omg TBC sucks, give back vanilla'
During WotLK: 'omg WotLK sucks, give back TBC'
During Cataclysm: 'omg Cata sucks, give back WotLK/TBC'
During MoP: 'haha Cata sucked didn't it, glad it's gone'
...Wait what? Combo-breaker! It actually speaks volumes as to the quality of MoP when there's barely any QQ on the forums about the expansion, and people prefer it to the previous one.
The thing is, Cataclysm gave us a lot of amazing things that really changed the way we play the game: reforging, the old world content update, transmogrification, 10/25 mans giving the same loot, guild achievements, and more. The problem is that it was very easy for people to latch onto the idea of 'lol rehash', when really... barely anything was getting re-used. Let's go through the list:
1. Old world content update: sorely needed. It's far better now.
2. Nefarian/Onyxia: completely different boss fight, just happens to be people we've already killed before.
3. Ragnaros: same as above. Also this boss fight is seen as a high quality fight in heroic.
4. ZA/ZG: ZG was COMPLETELY different, just happened to be set in the same place as the old raid. ZA was more similar to its previous version, but still had a few differences.
5. Dragon Soul: yeah, it had some bits set in Wyrmrest temple, but that's all it was. And it made sense that it was set there, lore-wise.
The stuff we got was high quality in Cataclysm. The problem was that there wasn't enough of it, and people focused on this dumb 'rehash' idea. It also kinda sucked that we ended on a fairly mediocre raid. Not as bad as people say, but it could've been better. But hey.