So that people that have tanks will actually play them. Why should we get incentive to play DPS instead? There's more than enough DPS out there. What the queue (and even party finder) pretty much always hurts for is tanks and healers. It's a way to reward dedicated tanks for being such. Don't get me wrong, I think that having endgame content only count toward the mount is kind of silly. Having duties like the relic reborn trials and such count toward it would help, even if the total amount of duties you needed for the achievement was raised.
The only thing I can think of is that CNJ was the only healing class upon release, so added incentive was given to play one. SCH only becomes at healer at level 30, and I don't even think Arcanist was a thing back then.
You took that entire thing way too personally. It's a stereotype, not a written in stone truth. Dragoons are often stereotyped as hopeless amateurs that spend more time on the floor than fighting the boss. Do I think that applies to all Dragoons? Hell no. In fact, I know some very skilled ones. Have I met some like that? Absolutely, but I've met plenty of derpy people from every class. Nobody's saying that the 'diva tank' stereotype applies to every single tank out there, but "tank entitlement" is a real thing. The fact of the matter is that a tank is way more likely to attempt to hold a group hostage in order to get their way than a DPS is. I mean, it'd be laughable if a DPS tried it (I've seen it, it really is). DPS ragequits halfway through the dungeon? Who cares, there's 85 more of them in the queue. If a tank does it, it really can leave groups waiting for 20 minutes for a new one.
In any case, I'm not even really sure why you brought that up here. It should've been left in the other thread where it belongs instead of derailing this guy's thread when he's making a suggestion to improve the tanking achievements.