Because electricity is the movement and flow of electrons, which are matter. Since they are matter, they cannot reach the speed of light.
By the 'speed of electricity' you presumably mean the...
Gravity is a state of space-time, however gravitational waves, predicted by Enstein general relativity yet not directly proven, travels with the speed of light, the highest possible speed, because...
The speed of sound will never be equal to the speed of light, except perhaps in some very weird materials which slow down the speed of light tremendously.