In vanilla we ran around in greens and some blues. Purples were a strange thing to see.
In Burning Crusade we were certain to get our blues and as the expansion moved along, we got our purples from heroics and Karazhan.
Wrath of the Lich King purples were standard attire. A blue was a filthy, awful thing. A purple was really just barely getting by anymore.
Cataclysm came and did it give us a new color to yearn for? Alas, no. It gave us blues! Blues! Filthy rotten poisonous blues, rather than yummy delicious purples, yes, we wants we do. Our... precious. Purples. But did we gets them? No! They takeses them.
As expectations progress, it is important to either keep up with the expectations or to change the change in expectations (derive the derivative's derivative!). Cataclysm failed to do this. Wrath of the Lich King was an effective teacher and it taught us that purples were something to have, not something to get. Perhaps Cataclysm could have successfully transitioned back to blues, but it did not. I blame some of this on the lack of sets; none except for PvP and a few crafted; neither of which are instance loot. I've said it before, that sets are special, telling players that they are allowed to stay in this gear for a while while they gather the rest of the set, that they aren't expected to get this item and drop it an hour later.
In conclusion: The title:
Cataclysm heroics should have dropped legendaries.