Electricity can't stimulate dead neurons. Muscles do compensate as much as they can for the loss of other muscles, as when abdominal muscles pick up some of the work of breathing. When there are no longer enough muscles to support a given movement, it's lost.
FES has been used to jump start regrowing fiber due to inactivity or a severed connection between the spine and the muscle, but in ALS the problem is the death of the motor neurons that control many muscles, not selective paralysis of a single muscle, as you might see in something like MS, where some patients have seen improvement with FES.
Also, spasticity and contractures are contraindications, because you are basically "making the frog leg jump," to be crude. The diaphragm pacer, which turned out to have negative effects for enough PALS that it no longer receives widespread use in ALS, is another example. Think about trying to charge a damaged battery.