Why? It just does, or perhaps God likes it that way.
Perhaps the Higgs theory MAY explain how ... but not WHY.
You've used the definition of a property to ask why ... and all it is is a label.
"inertia"
inertia is defined as the tendency of an object to continue in its state of rest/motion/direction. if u try to get down a running bus, your body was actually in motion with speed=the speed of bus. so it does not stop immediately u need to run for a while otherwise u will fall. this is inertia of motion.
Inertia of Motion
Inertia: the property of matter that resists changes in motion.
There are actually 3 kinds of inertia. They are as follows : 1. Inertia of Rest 2. Inertia of Motion 3. Inertia of Direction But nowadays people consider that there are 2 kinds of inertia , inertia of rest and inertia of motion.
Inertia resists the change in its motion/ velocity, and is proportional to its mass.
Inertia is associated with mass
Inertia
This is Newton's First Law of Motion. Objects in motion stay in motion because of inertia (refer to the law of inertia for additional help).
Inertia. Also described in Newton's First law of Motion.
the Law of Inertia state
Inertia is the tendency of an object in motion to stay in motion until acted upon by a force.