1. Warrior is the hardest tank class to tank with.
2. Tanking is the only role where you fight against your group instead of the mobs.
3. Tanking is the most misunderstood dynamic in this game. Chances are your group mate think they understand how you should be tanking but they don't. Thats why you hear things like "How hard is it to just target the mob and press taunt every few seconds."
4. The expansion/Blizz flawed character design. Before the expansion for most pve content it was not necessary to have points in Prot.....after the expansion for the majority of pve content after level 70 you are required to be prot either by the content or the group demanding that you be prot to tank.
5. Arms/Fury Before the expansion blizz seemed to have the goal of each talent spec being a viable alternative for all aspects of the game. IE they seemed to be trying to balance the chars with the concept that Fury Arms Prot should all be viable for Pvp pve soloing though some specs would be slightly better for a specific role than another.
Now after BC they seem to have completely shifted their logic and tune so that one spec will have a specific purpose and if you are viable in pvp you will have to respec to be viable as a tank and respec to be viable as DPS/support.
Biggest reasons:
MS PvP: more arena teams than tank slots in raids
It's a PuG: they don't need anything from that instance, so why incur a repair bill on your behalf. No need to deal with general PuG level idiocy for nothing.
1. No tank class is hard to tank with, if specced and geared appropriately.
2. So true - see "It's a PuG"
3. That's just a general problem cross-class, again see "It's a PuG"
4. Anything short of heroics can be tanked with no points in prot if you've got a small modicum of skill. Add heroics to the list as gear improves. Gone are the days of Sunderx20 afk tanking, and good riddance. It opened the door for more tank classes to be viable and in-demand. Can again see "It's a PuG"
5. Required specialization to fill specific roles. Nothing new there, it's just more pronounced. See any healing class, especially those that can fill multiple roles... oh wait, that's all of them.
I think I see a common theme here, something to do with PuGs. Yeah... that couldn't be why your group can't find a tank, could it? (not aimed at OP here, aimed at all the whiners complaining about a tanks shortage)