MS is caused by demyelenation of the nerve. The myelen sheath is what allows the electrical signal to flow along the neuron, while the neuron itself releases the neurotransmitter which tells the next neuron to either fire or shut down.

MG, on the other hand is caused by a lack of receptors to the neurotransmitter where the nerve and muscle meet.

The reason Mestinon won't help MS is that the electrical signal telling the nerve to release acetylcholine never reaches the nerve ending...lacking that signal, it just does whatever it wants to, ie. releasing neurotransmitters that either cause muscle spasms, or just shut down the muscle. But they have the receptors, just not the signal. Adding extra acetylcholine wouldn't help in this case.