What is the "reason" C is constant to all observers?

Bilge wrote:
Sure, it is. Scientists who are trying to determine
whether or not the speed of light is constant, will
certainly _not_ define it to be constant.

Actually, no one would ever try to measure the
velocity directly. There are a lot of things that depend
on the speed of light being constant which offer a more
precise result.

The most direct measurement is probably from binary
pulsar data.

Since a non-constant speed of light implies that the
photon mass is not zero and that charge isn't conserved,
you have two more ways to check.

brian a m stuckless