When a small metal pellet is dropped onto a block of ice, the immediate region around the area of contact melts and even steams. why is the thermal energy not shared instantaneously with the rest of the block to warm it as a whole?

The heat transfer in this event is through physical contact, there are other ways to transfer heat (radiation, convection etc.) but in this case. The channel or mode of heat transfer is through surface contact. Thus when the surface of the metal ball comes in contact with the ice block. Only portion is actually in contact.