The question of the speed of light has to do with the fundamental structure of the 4-dimensional universe.
Consider all objects, including a photon of light, to be 4-dimensional objects having very small X1, X2, and X3 measurements while extending some 10^13 miles along their 4th dimensions. These long time-like paths through the 4-dimensional universe are often referred to as world lines.
And along with that, consider the strange and curious way in which nature has worked out the instantaneous 3-D cross-section views of the universe experienced by observers moving about at various speeds relative to each other. The sequence of space-time diagrams below shows observers (blue coordinate inertial frames) moving at ever increasing relativistic speeds relative to the black rest frame (increase in clockwise rotations corresponds to increasing speed along the black X1 axis--time increasing along the X4 axis). The slanted X4 axes represent the world line paths through 4-D space while the X1 axes represent the instantaneous cross-section views across the 4-D space experienced by the observers (the 3-D worlds the observers live in at a given instant of time).
The photon is always represented by a world line slanted at a 45-degree angle with respect to the black rest frame. It always bisects the angle between the X4 and X1 axes for any observer (no matter what his speed, i.e., the slant of his X4 axis). Thus, the 4-dimensional photon has THE unique orientation among all 4-dimensional particles. It's angle-bisecting slant is unique--giving a ratio of dX4/dX1 = 1 in the inertial frame for any and all observers.
The arbitrary accidental calibration assignment of clocks (sec, min, hours, days, years, etc.) along the X4 axis results in our numerical value for c.
Why and how did nature manage to work out such a rule of rotating X1 axis to provide the 3-D world that an observer should experience? Who knows. But, one thing that is accomplished by that (in addition to having the same c for all observers) is that the laws of physics then are naturally the same in all inertial frames (no matter how X4 is slanted).