Because a vast majority of the player base does not care about dungeon running and only did it because of the rewards. I'd like to see some kind of chart with heroic activity since WoD started compared to other expansions, but realistically no way for us to know without Blizzard "giving/showing" us that.
I know everyone that I've talked to pretty much stopped running them after the first week and don't even run them on their alts.