The problem isn't just there aren't *many* tanks in queue (replace liberally with heals, it doesn't matter). The problem is that the reward is once they get their EoFs, unless they're fresh 80s, they have no interest in running a second dungeon unless it's for a friend/guildie/SO.
Not to mention, what happens the moment your dungeon group gets a DK "tank" which is uncrittable by resilience but with zero avoidance, attrracted by the asy emblems, AND your healer is barely able to queue (and thus *really* needs the dungeon?)
Finally, unless someone plans to get on top of the primordial saronite racket, he's going to get frost badges just to get his T10 and maybe one or two pieces from the vendor. If I'm getting enough emblems for my spec(s), I'm surely not going to despair if I skip my RDF once. If I'm getting twice the emblems from my daily heroic, it means I'll be done with it in half the time.
Anecdote, when I log in (druid) I queue for RHC as tank+dps, once I'm out I drop tank from the queue as I do my dailies (since i cba to switch spec every time). The moment I'll start farming for emblems again (gems, heirlooms) I'll might start requeuing as a tank once again, until then I just log off. And I'm a person who actually enjoys tanking dungeons, it's just not worth it.