Derah's comment hit the nail right on the head.
We romanticize the fact that TBC had so many dungeons, but many of them were very minor changes to the environment. If Blizzard did that now, you'd see a ton of backlash too.
"Wow, Blizzard, way to give us the same 4 instances using one basic model, are you really that lazy that all you did was change the mobs and props? Come on Bli$$!!!!1!1!!!"
People would argue that they were being uncreative, and the instances were uninspired. People would complain that it was not unique enough.
Honestly, is it bad that they were just same body different clothes? No. Just as it's not bad that they went with a less, but "more" methodology moving on from it.