While it'd be neat if warlocks were granted a 4th tanking spec unfortunately ...
It won't happen this expansion. It's simply too messy to try and balance a brand new tank mid expansion right after they added in another new tank. Maybe in Rise of the Murlocs.
People in general don't queue for a tank spot because they can't tank on a warlock. They don't do it because they don't like tanking. Adding more tank specs won't fix that. See Death Knights, Monks and a recentish GC twit.
There aren't that many warlocks right now anyway. Even if making warlock tanks would bring in more tanks hunters would bring far more people than warlocks.
I disagree on why folks don't tank.
Tanks are missing, especially in LFD because far too often tanks are treated like crap in LFD.
Once tanks are geared enough to get into LFR, then doing dungeons is a Valor acquisition choice...and a good one. It's just more tanking practice. And that can be good.
But as far too many dps and heals have made crystal clear, there generally just isn't room for learning and gearing tanks in the LFD. Enough that for any good groups a learning or gearing tank might get, the number of impatient or abusive groups will be exponentially greater.
How many times on forums here and in other places have tanks repeated the phrase, "I only tank for guildies"
There's a reason for that. Sure, a few tanks get heads too big for their helmets. They deserve the headaches they get from their swollen heads. But for the most parts, tanks take on the extra responsibility and extra focus and extra challenge and see it as a good thing and only abandon it (sometimes totally, other times only tanking in Raids) not because of the challenge presented, but because no one pays their subscription fee to sign up for abuse.
Unfortunately, too many players treat other players as if they are entitled to a pro tank in LFD so they can play however they want as if the tank isn't also a player who is also paying to play and that the game is cooperative.
Without LFD, it was harder, but the spirit of cooperation was MUCH, MUCH stronger because it had to be. You crapped all over the good tanks on your server, you'd never finish a heroic dungeon. Period. Now, with LFD, dps especially and sometimes heals and yes, sometimes tanks are free to behave as badly as they wish without consequence.
As long as so many players treat other players, tanks or heals or anyone, really... as Blizz employees who are beholden to them to provide them a service because they've paid for their subscription as if the other player hasn't also paid, then things like tanking shortages will ALWAYS be an issue.
I posit that the very day that the WoW community starts treating one another better, with more respect, compassion and humility, a HOST of issues including the dearth of tanks will rapidly vanish.