Why, among different metals, only zinc is favourable for the dehalogenation of vicinal dihalides?
Zinc is used effectively in the dehalogention of vicinal dihalides because a carbon zinc complex is formed in the mechanism which is probably more stable. This reaction is also known as Simmon' Smith reaction.
The stability and usefulness of the Simmons-Smith reagent (zinc metal) may be attributed in part to the higher covalency of the carbon-zinc bond together with solvation and internal coordination of the zinc (which makes the zinc complex stable). (Source)
Reduction of vicinal dihalides by zinc metal proceeds by way of oxidative insertion of Zn(0) into a C-X bond, producing an organozinc species (stable) which undergoes dehalogenation: