Why is this? To understand, you first have to take a look at the Warrior's tanking skills, and the Knight's tanking skills.
Knights have access to Aura Heal, Aura Shield, and their only redeeming ability, Shield Fortress. Their AOE is Grand Cross. Warriors get Tension Relax, Parrying, Endure, and Defender. Their AOE is Brandish Storm. Right off the bat, you can see that something is off. Warriors get 4 different skills that help them mitigate damage, while knights only get 3.
Lets look at the skills themselves.
Knights get Aura Hea 3/3: Cast the power of the Aura to a target and recover HP equivalent to Attack Power x 0.06 per overlapped Aura. 20 second cooldown.
Now, I haven't actually used this, but from what I've read online, it's an absolutely awful skill and not even worth investing points in to.
The warrior equivalent is Tension Relax 3/3: Suppress Rage and loosen tension to recover HP by the amount of overlapped Rage remaining. Max 60% for 20 sec. This skill consumes any remaining Rage. 120 second cooldown.
With 3/3 skill points invested, this gives the warrior a 60% heal over time. Now THIS is a useful heal. If it weren't for the 120 second cooldown, this ability would be game breaking!
5/5 points in the knight ability Aura Shield: Concentrate Aura on your shield for 10 sec. to decrease damage received by 20%. 60 second cooldown.
Alright! Something useful for Knights! I can definitely see this ability saving your skin in a tight situation.
5/5 points in the Warrior ability Parrying: Increase Parry Rate by 40% for 10 sec. 60 second cooldown.
Another potential life saver! Now this is what class balance should look like. Abilities that are different, but neither one is blatantly stronger.
5/5 points in the Knight's signature ability, Shield Fortress: Decrease damage received by 50%. Damage taken by all Party and Raid members within a 20m range also decreases by 25% for 10 sec. Cannot move or perform any other action while using Shield Fortress. 120 second cooldown.
Alright! This is probably the most powerful defensive cooldown in the game. Not only will it keep your butt alive, but nearby party/raid members get 25% damage reduction as well. I can't deny it. Once every 2 minutes, Knights get to be better tanks than warriors. Its only downside is that the knight has to stand still channeling the ability for its duration. This could be a problem if after you start casting, the boss decides to leave a nice pool of lava at your feet.
The Warrior counterpart is Endure. At 5/5: Damage taken is reduced 40% for 10 sec. 120 second cooldown.
It's still a good ability, but is simply outshined by Shield Fortress. Being able to move around and fight while it's active is a plus, but Knights win this round.
Let's look at Area of Effect attacks next.
Knights get Grand Cross 3/3: "Slam your sword into the ground releasing Aura on the surface to inflict 22% damage to a max. 10 surrounding enemies. This skill is most effective when used on more than 3 enemies, however, the effect is reduced when used on more than 6 enemies." 0 second cooldown.
Hey, it get's the job done, and it's definitely better than what Beastmasters and Monks have. One downside is that like Shield Fortress, you have to stand still while using this ability.
The Warrior equivalent is Brandish Storm 3/3: "Create a storm with your sword to inflict 32% damage to a max. 10 surrounding enemies. This skill is most effective when used on more than 3 enemies, however, damage is reduced when used on more than 6 enemies." 0 second cooldown.
Not only does this version do more damage, Warriors can move around while using it. This makes add control a lot easier and even makes it possible to kite certain enemies. Grand Cross literally has nothing on this ability. The Warrior version is better in every way.
Let's tally the scores.
Healing: Warriors win this one, hands down.
Aura Shield vs Parry: This one's a draw.
Shield Fortress vs Endure: This one goes to Knights.
Grand Cross vs Brandish Storm: Warriors, no contest.
So far it's Knights:1, Warriors: 2
To make up for not receiving the defense stat bonus of a shield(everything except the defense bonus is already compensated for via extra stats on 2 handed swords), Warriors get access to a fourth defensive skill known as Defender. With 5/5 points: "Defense, Dodge and Parry are increased 30% but Attack Power decreases 10%.
This buff lasts indefinitely until you cancel it. This instantly gives warriors the edge over knights in the parry and dodge department, but they'll still be trailing behind them in terms of flat defense rating.
Now, like me, you may be thinking "Ok, whatever, Warriors are avoidance tanks and Knights are flat defense tanks. I'm ok with that".
Because Defender is a scaling ability, there is actually a point in gear progression when a warrior tank will have higher defense(in addition to sky high parry and dodge) than equally geared Knights. Unless they decide to start putting absolutely ridiculously defense values on shields, this gap will only grow as new content is released.
This gear threshold has already been reached by many on the Korean servers where they've acknowledged Warriors as the kings of endgame tanking. Not because of self healing, not because of increased mobility, not because of higher DPS(and thus threat), and not because they value avoidance more than defense, but because of overwhelming superiority in all categories. The single exception to this rule is Shield Fortress.
Now all of this doesn't mean that Knights aren't a viable option. They are. Shield Fortress is a very attractive ability, and not everyone is interested in getting endgame gear. I just created this post so that people like me that were attracted to the "class with the shield" know what they're actually getting in to. I don't want people to regret their choice and have to start a new character(or worse, quit) once they find out the facts.
tl;dr version: Warriors are kings of endgame tanking. This isn't what I expected, and I don't want others to make the mistake of assuming that Knights will always have better tanking stats.