They seem to be fussy about soil conditions; there have been moles in exactly the same area of one of the parks in Oxford for at least 50 years, and never anywhere else. They're also, as has been said, very territorial. The young leave the mother, and live on the surface briefly until they either find a territory or die. If you get rid of your mole, it's unlikely to be too long before another moves in, if you have a healthy breeding population locally.