How come I have motor neuron disease, but nobody else in the family has it?

Most are nonfamilial. While there are some cases of motor neuron disease that run in families, the majority of cases occur spontaneously without any clear genetic cause. Hopefully, with time and continued research, we will be able to find the nongenetic reasons that cause some people to develop the disease while others do not.
Complicated. The causes of motor neuron disease are only partly known. There are some known genetic factors, but these only cause a small proportion (10%) of mnd. Most patients get the disease sporadically, i.e., without a family history. In there case it may reflect a combination of genetic factors which other members of the family don't have or some other cause. See: www.Alsa.Org for more info.