Easier to replace the energy powering the grid than to, say, power a vehicle directly with non-fossil fuel sources. It may still be using fossil fuels due to what is powering the grid, but that can be changed out over time; when using a gasoline powered car, you can't, until you replace the car.

It's why electric cars are viewed as being friendly; the grid they're plugged into may still be fossil fuels, but that can change at any time. A gasoline powered car won't change its reliance on fossil fuels until you replace it.