Wood is a natural material and its quality and performance can vary enormously. Generally, all wood used externally will be attacked by ultra- violet rays from sunlight and this will breakdown and kill the surface layer of the wood. To prevent this, the wood is treated with an oil, varnished or painted.
Unfortunately the ultra-violet rays will still penetrate most clear treatments so the best way to protect wood is to paint it, as was generally done from the early 17th century onwards.
The other main property of wood is that it takes in and loses moisture, and so expands and contracts regularly. This places a strain on any coating, as it must move and flex with the wood it is covering.