as the sparkplugs need to conduct a lot of electricle charge, over 20,000 volts, whoa!!!!!, throught them but remain insulated from the cylinder head you need a plug capable of these two contridictory conditions. So you need a plug with good conductivity through the electrode, this is the part that runs through the middle and is surrounded by a superb insulating material. when it comes to the spark the electricity can travel through a vacumm easier that it can travel through air. now as your cylinder is at max compression when the spark has to occurr this makes it relativly difficult for it to jump the plug cap and produce that much needed spark. which is why the voltage is so high. now in a diesel the compression ratio is much higher than in a petrol engine so the spark plug has to cope with much denser air; therefore you really need a fine outstanding plug such as NGK or MFI plugs.
CharlesY