By different metals do you mean different electrodes? The exchange current is a direct measure of electrochemical rate. The rationale is that, rate of reaction is faster at some electrodes than others, for the same reaction.

Why is the rate of hydrogen reduction faster at Iron surface than at Zinc?

You will have to review the electrode factors that affect electron transfer kinetics. Parameters like density of electronic states (which may not be a big deal for metals) is one. For some processes too, the mechanism of the electron transfer and whether or not the process requires adsorption/desorption, can be affected by the surface condition of the electrode. Besides, the catalytic involvement of the electrode is also possible.