When natural rubber is exposed to oil or chemicals, it swells, softens, and eventually disintegrates. If natural rubber is used where it could come in contact with oil or chemicals (oil hoses, grease guns, gaskets for oil pipelines, and just about any belt, hose, or gasket under the hood of your car), it's life is bound to be a short one. In the early days of automobiles, natural rubber was all there was, and rubber parts needed to be replaced on a regular basis. That is why neoprene, Thiokol, and Buna-N were in such high demand and people were willing to pay much higher prices. One part that lasts for four years is obviously worth the cost of four that only last a year apiece.
From Frank A. Howard, Buna Rubber: The Birth of an Industry. New York: D. van Nostrand Company, Inc., 1947.