This is a great question, and very difficult to answer.
Sound waves require a medium to travel through. That is, a mass of molecules. The speed of sound depends on the density of these molecules, which is why there is a different speed in water and air, and why there is no sound in the vacuum of space. Also, you notice how sound dissipates, which is the sound wave 'giving up' energy to the molecules it travels through.
Light on the other hand does not require any medium. It just keeps going through space at a constant speed untill it is absorbed. All of this lies in the theory of electromagnetism (light is an electromagnetic wave).
It is these properties (sound waves require a medium to travel in but light waves do not) and the fact that light waves are electromagnetic in nature that dictates their different speeds.
But to answer why they are different, opposed to how they are different, needs someone with a little more knowledge of electromagnetism than me! Steven Hawkins, a little help please...