They did it for the Demon Hunter and people went "oh well, if they can't think of anything unique, better to just go with 2 specs than to come up with a lackluster 3rd one"
Now 2 specs is the new standard. They have to put in less effort. Why should they design 3 specs if the community accepts 2 just fine?
There is no reason why there couldn't have been a 3rd spec at least. It's a dragon, that can be a ranged caster and a healer. There's absolutely room for a melee dps or a tank spec and especially considering it's a dragon race, that has been created by Deathwing, the leader of the black dragonflight.
People will object to this and say "we have enough melee dps and tank specs".
1. We also have enough ranged dps and healers. There was no need for any class at all. Classes are put into the game cause it's fun to have something new. Not because we need more of them.
2. The argument from people is that having more melee dps specs than ranged dps specs leads to having too many melees in a raid, and it gets really cluttered around the boss and raid groups already have to be designed in a way where they don't take too many melees.
But whenever the tank problem is being brought up and the old story comes up of Blizzard designing more tank classes in the hopes that that solves the tank shortage problem, people point out that more tank classes doesn't actually equal more people playing tank. Ok. I think that's probably true. So then what is the problem with for example having another tank spec on this class? Doesn't mean more people will play tanks. So the argument that "we have enough melee specs already" is completely irrelevant. It doesn't matter how many you have.
In reality people will play whatever they like, and if they like melee dps classes, they are already playing them.
If they don't like them, then they won't start playing them just because there's another spec.
And if a new class might entice some people to try out the melee spec, just because it's a new class and they are curious, then the original argument is flawed, and maybe a new tank spec could give us more players willing to tank. Which would be a good thing.
3. You literally lose nothing by getting another spec onto the class. Why would you be against that? It's more content.
No, we don't need another melee dps spec or another tank spec. We don't need another healer or ranged dps either. We don't need anything. But it surely doesn't hurt to have more. Especially when 2 specs for classes is becoming the new standard.
So please stop defending this and at least come up with good arguments. If you don't like tank specs, stick to the ranged specs of the class.