Simply because of the engine design and air regulation.
Diesel engine does not have a throttle plate (well it does but it most of the time it is always open, it partially closes to assist EGR etc. topic for another time) it does not control the amount of air coming in like in a petrol engine.
This means that diesel engine always draws in the maximum amount air, and power is regulated by the amount of fuel supplied. With petrol engine throttle plate controls the amount of air coming into the engine and thus due to the air-fuel ration variation the need for control.
The engine design from diesel to petrol is also significantly different. Running too lean on petrol engine let say 19:1 will cause significant raise in engine temperature and pre detonation. However, keep in mind that there are modern engines like the Mazda Skyactive-X that runs air-fuel ration of 30:1 at times and 90% of the time doesn't require spark plugs to ignite the mixture. Thanks to modern technology you could call this a hybrid between the two engines=).
Hope that helps.