Actually the speed of light can be different in different media, for example, it is higher than $c$ in Casimir vacuum and smaller than $c$ in a solid medium.

It seems the actual question here is why the speed of light in flat vacuum is the highest speed at which information can be transferred.

This comes from the Special Theory of Relativity where it was shown that if there was FTL information transfer, a casualty paradox would appear.

And now emerges another question, why exactly Special Theory of Relativity uses c in its Lorentz transforms rather than any other speed. This is due to the idea that all inertial frames should be indistinguishable: the speeds of all processes, either electromagnetic, mechanical or gravitational should change similarly when changing a reference frame. If electric processes changes one way and say gravitational another way, we could determine whether we are in a moving or stationary frame.

It follows that all fundamental interactions should propagate at the same speed for this criterion to be met.

So actually if there somewhere exists a medium where speed of light is higher or smaller than c, then in such medium it is possible to determine whether the given frame is absolutely moving or stationary relative to the medium. Such medium does not have the main propertiy of our vacuum, which distinguishes it from Ether: that there is no difference between moving(without acceleration) in it and being in rest.

Actually the speed of light can be different in different media, for example, it is higher than $c$ in Casimir vacuum and smaller than $c$ in a solid medium.

It seems the actual question here is why the speed of light in flat vacuum is the highest speed at which information can be transferred.

This comes from the Special Theory of Relativity where it was shown that if there was FTL information transfer, a casualty paradox would appear. This is because $c$ is used in Lorentz transforms.

And now naturally emerges a further question, why exactly the Special Theory of Relativity uses $c$ in its Lorentz transforms rather than any other speed. This is due to the idea that all inertial frames should be indistinguishable: the speeds of all processes, either electromagnetic, mechanical or gravitational should change similarly when changing a reference frame. If electric processes changes one way and say gravitational another way, we could determine whether we are in a moving or stationary frame.

It follows that all fundamental interactions should propagate at the same speed for this criterion to be met.

So actually if there somewhere exists a medium where speed of light is higher or smaller than c, then in such medium it is possible to determine whether the given frame is absolutely moving or stationary relative to the medium. Such medium does not have the main propertiy of our vacuum, which distinguishes it from Ether: that there is no difference between moving(without acceleration) in it and being in rest.