Question

why do you different inertia of different bodies

Answer 1

because moment of inertia depends upon mass of body , distribution of mass of body , and position of axis of rotation .if body varies with their mass or distribution of mass then inertia also varies .hence different body has different moment of inertia.

Answer 2

Inertia of a body depends on its mass.. the more the mass the more amount of inertia in the object...
If we take an example of a cricket and a ping pong ball, the cricket ball will have more inertia