Short Answer: to avoid conflict with his human neighbors. In China, India and Africa wild elephant populations are increasingly crowded by human activities. Defending their crops, villagers barricade themselves in - and the elephants out. Elephant habitat is broken into isolated pieces and herds are separated. But resourceful Foundations and NGO's working with land owners and local government are providing safe passage linking preserves and wilderness areas. Sometimes a land corridor - but increasingly a tunnel or overpass. The latest is a 9-mile fenced-in corridor complete with a highway underpass on the slopes of Mt. Kenya. You can read about it here.
For over 50 years the remarkable David Sheldrick Wildlife Trust has been saving elephants who wander into trouble - one at a time. The profiles in their Orphan's Project read as well as any children's book about helpless baby animals. They successfully return these lucky ones to wild herds. This is a shinning point of light in our troubled world.