Here's why:

* Ethyne has a triple bond: The carbon-carbon triple bond in ethyne is highly reactive due to the high electron density and the presence of pi bonds. This triple bond makes ethyne susceptible to electrophilic attack, leading to addition reactions.

* Benzene has a delocalized pi system: Benzene has a stable, delocalized pi electron system that makes it less reactive than ethyne. This electron delocalization helps to stabilize the molecule and makes it less susceptible to attack by electrophiles.

Reactivity Comparison:

* Ethyne: Reacts readily with halogens, hydrogen halides, and water to form addition products. It is also used in combustion reactions to produce a hot flame.

* Benzene: Requires specific conditions (e.g., catalysts and high temperatures) to undergo electrophilic aromatic substitution reactions. It is relatively unreactive towards most common reagents.

In summary: Ethyne's triple bond and lack of aromatic stabilization make it significantly more reactive than benzene.