A commonsense argument.
We know that the speed of sound is a function of the characteristics of the media (air, water, metal, wood, etc.).
We know that the speed of light is a function of the characteristics of the media, for instance, an atom cloud it may travel through, as stated in the article Mitchel posted. An example is “slow light”.
Similarly, we should consider (as many have) that the speed of light in a vacuum is a function of the characteristics of the vacuum media, i.e., ether. There are many strong arguments and demonstrations for the existence of ether, and Einstein himself has not discounted ether. Ether is the most obvious and simple argument for the speed of light.
Philosophically, we can assume that the universe consists of infinite levels of smaller particles, each representing sub-universes of unknown physics. So for ether to exist, it is just a matter of one of those sub-universes possessing the physics of “ether”.
But why is the speed of light that number... -Mitchel Cohen