In Standard Hydrogen Electrode why only Platinum electrode should be used and why should it be dipped in HCl?why can't we use any other metal dipped in any other acid other than HCl?
Not just a platinum electrode, but electrode covered with so called platinum black - that guarantees very fast reaction (so the potential is not changed by the reaction kinetics). Cl- is just a very good, inert counter anion, with reasonably high diffusion coefficient (so the solution has as low resistance as possible).