Good walnut certainly looks nice, but it does have its disadvantages on a very accurate rifle, particularly if it is to be used in all weathers.
Being a natural material, even the best wood absorbs water in damp conditions, then sheds it when the weather is warm and dry.
This can cause tiny movements, which can upset accuracy.
Most plastics, particularly those incorporating glass fibre, are not so affected, and retain their stability in all conditions of heat, cold and humidity.
This ensures constant accuracy once the rifle has been properly zeroed to your chosen load.