why does the speed of light in vacuum never change? Under any circumstances, The speed of light in vacuum never change, why?
The simplest explanation: Light consists of photons, sub-atomic particles of light. These photons have no mass. Special Relativity tells us that any massless object has to travel at the speed of light. It cannot travel at any other speed whatsoever. That is why the photons travel only at the speed of light (in vacuum), and no other speed.
That is an assumption. Every checkable prediction can be checked right. Then we believe that assumption is right until any phenomenon break the prediction from the assumption. The more fundamental assumption is that electromagnetic laws, Maxwell equations, do not change under any inertial reference system. Max equations can lead to the invariance of light speed.
In the absence of extremely large concentrations of matter and/or energy in the vicinity, the electromagnetic constants of the vacuum (its permittivity and permeability) give rise to the speed of light that we measure by the equation
(speed of light) = (1/(square root of (permittivity x permeability)))
So saying that the speed of light does not vary from place to place or from time to time is another way of saying that those constants don't. Physics does not furnish us with any mechanism for doing so; you'd have to invent one entirely from scratch yourself using magic.
Another way of looking at this is to notice that in Einstein's equation
(energy) = (mass) x (speed of light)^2
if we let c vary with time or location, then E varies, and energy is no longer conserved. Energy conservation holds for the universe we inhabit, which implies c isn't going to change.