We know that, petrol is ignited by spark and diesel is ignited by compression ignition, the volatility of the petrol is greater than diesel. The spark plug produces the spark only at some places, not at all the point of air-fuel mixture. If the air-fuel mixture from the carburetor is completely vapourised the fire produced by the spark can penetrate throughout the mixture to burn all the mixture. Hence to produce the complete vapour of air-fuel mixture, the volatility of the fuel should be more.
Since the volatility of the petrol is greater than diesel, if we use diesel on petrol engine, the carburetor cannot produce fine vapourised mixture of air-diesel due to low volatility of diesel, hence this improper mixture reaches the combustion chamber. Now at the end of the compression stroke the spark will be produced, this spark only will burn the diesel where it is produced, the rest of the mixture will not receive the enough heat from fire produced by the spark for burning and the mixture will remain as a unburnt mixture tend to various efficiency loss, this is due to the improper penetration of fire. The improper penetration of fire results from improper vapourisation of air-diesel mixture and it results from low volatility of diesel.This is the reason for Why diesel cannot be used in petrol engine?.