The population charts show active toons.

But what is active? census+ (warcraftrealms) considers a toon active if it logs in and is spotted once a month. With so many people using Census+, even a single short login once a month on many servers means getting spotted.

In my guild we have quite a few DKs in the late 70s to 80. But among 20 raiders, there are 2 active DKs and a third (my own) that is semi-active in the raids (I use my DK on farm content, and my warrior on progression).

Several of the DKs in the guild are just bank alts, crafting alts, or raid alts (if a DPS or tank fails to show).

DKs do make pretty solid tanks and DPS, but they actually do have to be learned in order to do so.

Anyone can turn out decent DPS with a DK, but doing so without gimping the threat table, and going from decent to good - its the same as it is for hunters, you can faceroll and be decent, but being good takes skill.

Likewise DK tanking. Very basic content can be easily tanked, but the DK very quickly runs out of tricks. After a while it can plateau. A more skilled warrior player still makes the best tank - that class can go the farthest, but only at a skill range most players cannot attain. At average or good skill, the DK shines. For a top skilled player, the DK will be the weaker tank, though its unlikely anyone but that player will notice once the player hits that level of skill.

That said, I'm -not- a top skilled player, but I'm already seeing the limits of DK tanking. I'm more in the range where I can see their limits and advantages, can see that my warrior could match every single one of those advantages, but have yet to be able to push my skill enough to get there. I can see the pot of gold at the end of rainbow, but I haven't yet figured out how to get to it.

Eventually I will hit a point where my skill exceeds the ability of the DK class to keep growing, but the warrior class (which is exceedingly weak at early human skill ranges) will keep giving me options.

I guess that means the DK makes for a great starter tank. Like the paladin. Anyone can play a decent or good paladin tank, but no one can play a great one - the hard limits of the class hit before the human limits are reached. Only unlike the paladin, you can skip 55 levels.

Personal opinion of Skill to output:

DK / Hunter / Paladin:
1 - 2 - 3 - 4- 5- 6 - 7- 8 - 9 - 10
3 - 4 - 5 - 5 - 6 - 6 - 7 - 7 - 7 - 8

Warrior:
1 - 2 - 3 - 4- 5- 6 - 7- 8 - 9 - 10
1 - 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 10

Almost every other class:
1 - 2 - 3 - 4- 5- 6 - 7- 8 - 9 - 10
1 - 2 - 3 - 4- 5- 5 - 6- 7 - 8 - 9


IE: classes that are easier to be average or good in are going to be more popular, even if at the top end player range, they stop delivering.