Leather. Tradition. Nothing wears like a leather saddle, nothing has the look, the feel, the scent, the class, of real leather. A good leather saddle will last a life time and can be passed down - synthetic will never have the same feeling.
A good leather saddle may weigh 40 or even 45 lb. While a synthetic only ~20. The 20 or 25 lb difference doesn't make a difference on the horse.