To extend the life of wood; exterior wood should be treated for weather resistance. Moreover, treated wood lasts longer when sealed by the application of waterproofing compounds. Some prefer a painted wood fence to wood in a natural color. All of these extra steps cost money and require more work to accomplish.
Painting a wooden fence lasts only for a short time. Exposure to the elements of sun and weather causes paint to fade, crack, or peel. This requires removing the bad areas with a scraper, then sanding, priming, and then repainting. Many have to repaint fences every year, which is not very convenient.