Why can't I write an implicit operator from a Base class to a Derived class in C#?

public class Derived : BaseClass
{
    public Derived(string name) : base(name) {}

    public static implicit operator BaseClass(Derived derived)
    {
        return new BaseClass(derived.ColorHex);
    }

    public static implicit operator Derived(BaseClass baseclass)
    {
        return new Derived(baseclass.name);
    }
}

This won't work. why isn't it allowed?
I can potentially write the logic necessary for it to make sense, especially when converting from the base to the derived one.

EDIT:Changed the title of the question