Here is a classical picture that gives a qualitative feel for why light would be slower. That is not to say that the mechanism here is right. This is actually more an explanation of why sound travels slower than light. But assuming light interacts with atoms and this takes time before moving on to the next atom, you can see enough similarity.
Light travels at the speed of light. So do the electric and magnetic forces between atoms in a solid.
Suppose you have some sort of disturbance that travels from atom to atom in a crystal. The disturbance might push an atom out of place (sound), or distort its electron cloud (light) or something else.
The news that an atom or electron is displaced travels at the speed of light, so neighboring atoms begin to feel a change in forces that quickly.
But neighboring atoms/electrons have mass. They begin to accelerate when they begin to feel the changed force. It takes a while before they have been displaced as far as the original atom/electron. So the disturbance propagates slower than light.
We can do a really crude approximation to check on this. We suppose that sound propagating through Copper displaces atoms, while an electrical pulse displaces individual conduction electrons. We suppose that the electrical forces holding atoms in place are about as strong as the forces holding electrons in place.
An proton or neutron is $1800$ times more massive than an electron. A copper atom has about $63$ protons and neutrons. It is therefore about $110000$ times heavier than an electron. We might expect the acceleration of a conduction electron from an electrical pulse to be about $110000$ times greater than the acceleration of an atom due to a sound pulse. We might expect the speeds of propagation to have a similar ratio.
The speed of sound in a metal like copper is ~5000 m/s. (Depending on factors such as if the copper is rolled or annealed. See The Engineering Toolbox.) The speed of an electrical pulse in a copper cable is $40$% to $75$% the speed of light in a vacuum. See Wikipedia. So an electrical impulse is actually $24000$ to $36000$ time faster. This is in the ballpark of what we might expect.