Question

Why potassium nitrate is not used for laboratory preparation of oxygen?

Answer 1

BECAUSE IT MAY CAUSE AN EXPLOSION
Laboratory preparation of oxygen from pottasium nitrate involves heating of pottasium nitrate.
Potassium nitrate itself is not combustible, it is considered to be a strong oxidizer, meaning that it releases oxygen when it breaks down, or decomposes. This oxygen release greatly enhances the flammability of other materials. Potassium nitrate also emits heat when it decomposes, so it may provide a source of ignition if stored with flammable materials. Potassium nitrate also gives off toxic fumes when in a fire.