That's just my opinion of it. The question is asked over and over and sometimes in different ways its asked. People sometimes go in LFG: Need Tank, can't find Tank (Healer), We really need a Tank (Healer), been looking for 3hrs, Please we just need a Tank and we're good to go (gtg). So WoWInsider had a short article asking if adding the another new tanking class (Death Knights) going to solve the tank class shortage. My Answer is No! What you may end up with is a lot of new elite tanks in Death Knights maybe.
As a tanking class and person that's unguilded I spend all my time in LFG so I know and I hear those cries for needing a tank all day long. One of the things in playing WoW is that the people are real people vs all NPC's. Thus people of all kinds can be found to be contrary to the normal as: idiots, jerks, inconsiderate, selfish sometimes and some just don't try to learn how to play their class properly. Dealing with a lot of those people are just not worth putting up with sometimes as a tank even in PuG's. I've flat out walked out of PuG groups as the tank. Lot of Tanks choose the safety net of their own guild rather than helping out PuG's. The thing I often hate the most is getting stuck with the high cost of repair bill or due to DPS. Getting a big repair bill and no loot doesn't always sit well when you loose roll after roll on shards either when no one can use the BoP loot. When it comes to soloing as a tank that's not a problem to me yet everyone is different.
Most people just rather sit back and easily DPS its no stress for them. I do like the role of defending my peers. Tanking can me fun and stressful all at the same time. Tanking is a Thankless Job. There are times when I just choose not to respond to a LFG call to come lead a group through a instance. Often I respond more to a friend call with a good group to do a instance run or people I've grouped with before unless I really need the run. There are times when in Shattrath city that I can clearly see a few tanks around and a dozen healers of all kinds sitting on their mounts doing what seem to be nothing. Yet you can hear in LFG people are clearly asking for healers and tanks for quite some time for a instance. There is a reason for that. Healers don't like getting killed. Decent tanks don't like people telling them how to do their job and dealing with idiots, people that can't control their dps/aggro or start fights before the tank is even ready. I've been in lots of groups where people start fights and I'm still drinking for mana and not ready. That irks me.
Often if the group fails somehow everyone seem to point fingers at the tank somehow more so that at themselves or failing to CC a mob. Yes sometimes that could be valid to blame a inexperienced tank, but then again person is inexperience. It takes a coordinated group to get anything done correctly or efficiently. How often do DPS blame themselves in a 5 man or for been afk in middle of fight? Its my experience in 5 man PuGing that VERY FEW people have Threat Meters installed thus easier to pull aggro. I'm a tank I see it because I have both a threat/dps meter. Someone sometimes pull aggro and my job gets harder and as a mana user I have to burn even more mana when someone pulls aggro. If you got poor DPS its hidden in the group. If you got a poor tank its obvious and he dies group wipe. A poor healer its obvious and tank dies group wipe and then group blame the tank for dying. Tanks take a lot of crap period.
Who needs that in a game that should be fun. If your decent tank you get asked to join a guild or hopefully get put on other people friends list as a alternate to their guild tank. I get asked allot to join guilds but I refuse of my own choice and choose to be guildless as a tank for now. In the end the tank gets stuck with a 2 digit gold repair bill, as I've seem many of those and I think to myself all my profit is gone. I have to make that back up somehow. Tank repairs are allot more than anyone else in the group and more likely more than everyone combined for repairs even if you wipe a few times in 5 mans.
Most people have seeing big numbers syndrome and crits so they love to DPS and they vastly out number what you see tanking or healing. When I'm tanking often I'm not really looking at numbers I'm busy enough doing my job and what I'm managing is generating enough threat to keep the mob CC glued to me or managing cooldowns. So I'm not big numbers happy. I just show up and do my job as a tank. My belief is that tanks and healers are both more so selective in the instance they choose to do and with whom. Often they prefer their guilds vs to group PuG with. If a tanking class has nothing to gain from a Instance run they almost have no good reason other than gear or faction reps to come or volunteer to go tank for a PuG doing that instance. Thus you have a certain level of players that just wont respond to a group in LFG needing a tanking class nor put up with a lot of stupid stuff people often do to cost them extra repairs. So having Death Knights won't solve that problem from my point of view.