Question

why the fire caused by electric short circuit should not be extinguished by pouring water?

Answer 1

The main reason is the risk of electrocution.

Water is conductive in nature and hence the electricity in the equipment could reach the extinguisher (the person dousing the fire using water) and can electrocute him/her.

Answer 2

Since water is a conductor of electricity when water is poured on the fire caused by an electric short circuit, the water may conduct the electricity and the person pouring water may experience an electric shock or even may be electrocuted.

Carbon dioxide or other bad conductors can be used for putting out the fire.

Hope that helps!