Because the normals of the invisible areas are reversed. The mesh front faces are pointed into the model and need to be flipped to point outward. This is usually caused by split models where Blender can't determine which way is inward and which way is outward and depends on how the mesh was built. Another cause is by using a scale x type command that causes stuff on one side of the mesh to be flipped to the other side. A lot of ppl don't realize that this method also flips normals.