Explain why diesel engines don't use spark plugs.

diesel engines do not use spark plugs because diesel has more oil, burns slower, and is more difficult to ignite. Therefor spark plugs are insufficient with just spark so instead they use glow plugs witch provides a high amount of heat to the fuel and ignites more quickly and reliably. 

The amount of compression pressure in a diesel engine is sufficient to produce enough heat to ignite the fuel. The glow plugs are only used on a cold start.