Why doesn't the CLR always call value type constructors

Question

I have a question concerning type constructors within a Value type. This question was inspired by something that Jeffrey Richter wrote in CLR via C# 3rd ed,

Answer 1

Another interesting sample:

   struct S
    {
        public int x;
        static S()
        {
            Console.WriteLine("static S()");
        }
        public void f() { }
    }

    static void Main() { new S().f(); }