Por que só dragonglass / fire é capaz de destruir White Walkers?

Why is only dragonglass used to destroy White Walkers?

Não é, nós temos duas maneiras conhecidas de destruí-los, com uma possível terceira opção.

Dragonglass

aço valiriano

Também se especula que o fogo do dragão pode matar os Caminhantes Brancos, mas não sabemos se ainda é possível.

Também sabemos que o fogo normal não pode matar White Walkers, pois as chamas parecem passar quando eles se aproximam.

Why do only dragonglass (and fire) have the power to destroy the White Walkers?

Quanto ao fogo, sabemos que não pode matá-los, mas pode matar as criaturas. Isto é o que Jon estava falando em sua demonstração para Cersei, não para os White Walkers. Se isso não ficou claro, o script oficial deixa claro:

The Hound chops an arm off at the elbow. The arm keeps flexing and grasping.

The demonstration shakes Cersei in a way few things can.

And though Tyrion is seeing this for the first time as well, he spends more time watching Cersei than he does watching the wight itself. Noting her reaction.

Everyone who has never seen a wight is shocked as well.

Everyone except for Qyburn, that is, who steps forward to pick up the still-moving hand and forearm, and beholds it with fascination.

With a flint, Davos sparks a torch and hands it to Jon.

He takes the hand from Qyburn, ignites it and drops it. The hand dies.

JON: We can destroy them by burning them. (beat) And we can destroy them with dragonglass.

He draws a dragonglass dagger.

JON: If we don’t win this fight, then that...

He points to the wight’s torso as it crawls toward him with its single arm.

JON: ... is the fate of every person in the world.

When the wight’s torso reaches him, he picks it up by its remaining wrist, and stabs it with the dragonglass. It deanimates and falls to pieces, ending the demonstration.

_{Game of Thrones, Season 7 Episode 7, "The Dragon and the Wolf" - Official script}