The problem isn't the amount of classes that can tank, its a deeper issue then that. I could write a fifteen page paper discussing the reasons behind the long LFR queues, but i'll sum it all up here.
-All the good tanks have guilds who clear normal/heroic weekly, they have no reason to dip into LFR anymore (and frankly why would they?). All the competent but under geared tanks do flex instead of LFR because it eliminates the freeloaders and people who refuse to perform at a baseline level (dps doing what i did as a tank in dungeon heroic blues im looking at you). As an extension of this, next expac you'll see drastically lower que times because the first patch of the first expansion tanks will be looking to gear again by any means. This will taper off further into the expansion. As a final note, why would a tank geared through normal sit through LFR "progression"? LFR sometimes takes more time on my healer alt to complete a wing of, then it takes us to clear to paragons. (that's one night of 3 hours, 13 bosses, its on normal ofc but still).
-LFR is hostile, ignorant, and annoying. As a tank, most of us take a semi-leadership role, the best of us also take pride in knowing nearly everything we need to know about a raid or dungeon. So when people come in direct conflict with ethier us, our friends, or innocent bystanders it causes headaches that are worth avoiding entirely.
- You can't freeload as a tank ever. If you're caught slacking or going afk you directly affect how a raidfinder will go and at what speed. You get called out really quickly. This is why i feel alot of people don't tank RF. Tanking isn't hard, having the extra knowledge to excell takes a bit more work then other classes "maybe" but tanking itself is easy.
-This is more personal preference but: I hate other tanks, period. You know all that stuff i was saying about taking pride in knowing my class, encounters, etc? Well, heres the negative side of that. Other tanks who are clearly slacking, dragging ass, don't know fights and even worse don't ask? big problem. If my co-tank, off-tank, what have you can't communicate and also has no knowledge on how the fight goes? it really bothers me. Worse, are tanks who don't know how to play their class at all. you can honestly list any tank in the game, tell me that they are "squishy" and i can 75% of the time tell you why. So how is a tank being squishy important to my arguement? Well, if i can pick up why other tanks are squishy just by talking to other players and glancing at my skada meters, then their is absolutely no excuse on why a tank can't quickly do ten minutes of basic class research to suck slightly less in RF. This solves two fundemental problems. Firstly, it lessens the chance that a new tank will have a bad experience in LFR, increasing the chance that he'll come back in. Secoundly, it makes more experienced tanks more likely to notice the effort and appreciate it. This will cycle fresh blood into the queues and reduce wait times.
- I think it fundamentally boils down to the simple fact that dps are competitive amogst one another, so if a dps is doing shitty dps most of the other dps don't really care all that much because they're competing for the tops slots. However, tanks have to work together and carry an equal load, when the other tank doesn't carry his share, hes letting the entire group down and that's a headache i'd rather avoid. Healers work alot like dps, in that they are competitive the difference being that any healing that isn't getting done is generally picked up by healers who are competing to do as much healing as they can, which honestly, is awesome design.
TLDR: Hate to break it to you bub, but LFR just isn't appealing to tanks with flex in the game. Blizzard can't fix the will of the available tanks. LFR players have to look inwards to find the solution instead of pointing at the lack of tanks.