Petrol is a flammable hydrocarbon, and it can burn in air if ignited. Water is a stable compound of hydrogen and oxygen, and its chemistry is such that it will not burn. Looked at another way, the...
Water is heavier then Oil, so it goes to the bottom. Once there it evaporates and pushes the oil out into the air. As the oil goes though the air it sucks in more oxygen and explodes.