I shot a pile of pigs in central Idaho 30 years ago. They were making a pretty concerted effort at getting rid of domestics that went feral. The hogs I shot were on old ranches that had old apple orchards on them.

What stops them in many areas is heavy snow and lack of food source. They really can't make it in stands of continuous evergreens or areas that get deep snows for many months on end. Medium amounts of snow and a good source of tree nuts like hickory or oaks, and they do just fine.