Simply because it's Special Attack isn't that much lower than it's Attack. You can have success with a Special Zeraora, it's just used less, I believe.
It's not really a wall; but as mentioned before, Calm Mind Zeraora could be used to up it's Special Defense.
However, Zeraora has one of the worst special learnsets I've ever seen. The only notable moves are Thunder, Volt Switch, Grass Knot, and Aura Sphere/Focus Blast.
Those are the only reasons I could come up with.