why do we use the mass of object rather then its weight

Réponse:

because the object's weight is the gravitational force on it and this will change if you move it to another planet or get further away from the Earth. However, the mass will never change, its a fixed property.

In scientific langage, weight is the force in Newton.

The mass is material contained in an object. It not depends on the place where we are. The mass mesures with a scale and expresses with kilo in physical or in gramm in chemistry.