When light travels through a material, it gets "slowed down" (at least its net speed decreases). The atoms in the material "disturb" the light in some way which causes it to make stops on its path. This is expressed in the material's permittivity and permeability: Its ability to transmit electromagnetic waves.
But then what causes the limited permittivity and permeability of vacuum? There are no atoms there to disturb the light, so what is keeping it from travelling at infinite speed? Is there a special property of space itself which determines the speed of light? Or is it the virtual particles in vacuum which interact with the light?
Best Answer
What causes these constants to have the values they do is simply our choice of a system of units.
When you have a unitless constant, it makes sense to ask why it has the value it does. For example, two of the lines in the visible spectrum of hydrogen have wavelengths in the exact integer proportion of 28/25. When this was first discovered, it made sense to ask why it had this exact value, and the answer was unknown. Later the answer was discovered. Similar considerations apply to other unitless constants of physics such as the ratio of the masses of the proton and electron, or the fine structure constant. There is at least in principle some hope of finding some future theory of physics that can explain their values.
There can never be any such explanation for the value of a constant that has units. That's because the units themselves are arbitrary. Here is a nice discussion of that in relation to the speed of light: Duff, 2002, "Comment on time-variation of fundamental constants," http://arxiv.org/abs/hep-th/0208093