why the present day surge diverters use non-linear resistance elements

Non linear resistor has the property to block the current upto a certain level to which it is set i.e. it will block the current from passing through upto a preset value and allows the current to pass through it when current exceeds( as in case of lightning surge ) , thus grounding the lightning surge and hence prevents that large current from reaching the equipment which is to be protected.

Best protection for cable TV is a wire from that cable to earth ground. Other incoming utilities (ie telephone, AC electric) cannot make that same low impedance connection. So a protector makes that connection. Non-linear means each wire remains unconnected in normal operation. And connected low impedance to earth during a transient. Cable has better protection. But non-linear resistors are required so that telephone and AC electric service is not shorted out by the earth ground connection.