Enough classes can be tanks to mean it shouldn't be a class problem.
Yet a raid requires only TWO tanks but SEVEN healers. If the shortage was because of classes, then it would effect healers more than tanks.
If there is a problem with DPS queuing 45 mins for a raid, then I'm not sure it is a problem Blizzard can easily fix, except for making it attractive and lucrative for geared tanks to do 'old content'.
Heres a few simple reasons:
1) THE TANK MUST KNOW TACTICS: It is expected that both tanks know the tactics for each and every fight. To a large degree it doesn't matter if healers don't know the tactics as long as they heal, or DPS know the tactics as long as they DPS. There is more onus on a tank to know what he is doing.
Notice a trend where wipes happen a lot in LFR? It is on fights which require even DPS and Healers to know boss-specific tactics.
2) THE TANK MUST BE GOOD AT LEADING: In a lot of LFRs the tank is the one that leads the way. Yes some raids linear, but sometimes you have a room full of trash and 'the route which we always take when doing this raid' is not obvious, and people get nervous if they don't know it themselves. What trash is usually skipped? Will people shout and moan if I do it wrong?
3) YOU ARE IN THE SPOTLIGHT: Random DPS barely get noticed unless they do something terribly wrong or afk. Same with healers. With tanks you are in the spotlight. Everyone knows who is tanking and all eyes are on you whilst you are doing it. People get worried about making mistakes, but it is made even worse for tanks because you are in the spotlight and if you cock up everybody knows about it. If you aren't confident, this can be very off putting.
I'm sure there are other reasons as well but if you look at what I've listed, the reason people don't queue as Tanks is because of SOCIAL FACTORS, which is nothing Blizzard can change.
Tanks need to know the boss mechanics because ultimately its them who are positioning bosses and reacting to a lot of what happens. Tanks need to be good leaders and be able to handle being in the spotlight. Not everyone is cut out for this - especially in the toxic environment of LFR. It is much easier to tag along as DPS or Healer and not really understand the tactics, and just fade into the background.
The only way we are going to get more tanks in LFR is either a) bribery or b) a complete rework of the necessity of a tanking role in LFR. Or of couse c) DPS just have to learn to deal with it.
I say this as someone who always does LFR as a tank.