Question

Petrol also does not react with sodium or potassium. But it is not selected for preserving sodium or potassium. Why?

Answer 1

Sodium and Potassium metals are highly reactive .

Sodium reacts with moisture present in air to produce Sodium hydroxide and H2 gas.

2Na +2 H2O ----> 2NaOH + H2

Its a exothermic reaction. Sodium metal burns vigorously with yellow flame. 

Hence to avoid contact with moist air , sodium and Potassium are stored in Kerosene.

Well why only Kerosene :

---> Petrol and kerosene both are obtained from fractional distillation of fossil fuel petroleum.
B.P of petrol is : 95 °c
B.P of kerosene is : 150-300 °c
So petrol evaporates faster than kerosene.
Both can be used but kerosene is selected as it is cheaper and readily available than petrol.

Answer 2

well, petrol have quite low boiling point than the kerosene and can evaporate easily. and also the kerosene is cheaper than petrol.
The potassium as well the sodium are very reactive metal and can instantly react with the air moisture resulting in a exothermic reaction which can start fire. So in order to protect them they are preserved by the kerosene in order to avoid their contact with air.

Hope this will helps.