Because sodium catches fire in normal air .

Explanation:

Storage of Sodium:
Sodium is a highly reactive metal which can catch fire or explode if exposed to air or water. Therefore, it is not safe to store sodium in its pure form. It is usually stored in kerosene because kerosene is an inert liquid which does not react with sodium. When sodium is stored in kerosene, it is isolated from air and water, and hence it remains safe.

Storage of Phosphorus:
Phosphorus is a highly reactive non-metal which can catch fire if exposed to air. Therefore, it is usually stored in water or under a layer of water to prevent it from coming into contact with air. When it is stored in water, it does not react with air and remains stable.

Storage of Magnesium:
Magnesium is a highly reactive metal which can catch fire if exposed to air or water. Therefore, it is usually stored under a layer of oil or inert gas to prevent it from coming into contact with air or water. When it is stored under oil or inert gas, it remains stable and does not react with air or water.

Storage of Zinc:
Zinc is a moderately reactive metal which does not react with air or water at room temperature. Therefore, it can be stored in a dry place without any special precautions. However, it should be protected from moisture to prevent it from corroding.

Conclusion:
In conclusion, sodium is stored in kerosene because it is a highly reactive metal which can catch fire or explode if exposed to air or water. Kerosene is an inert liquid which does not react with sodium and hence it is safe to store sodium in it.

Because sodium catches fire in normal air .
This is because it is highly reactive and has a tendency to form ionic bond by losing electron, which makes it combine with oxygen in air to form Na2O, along with the production of heat, the reaction we call as combustion.