Why is List/IList not covariant?

Question

.NET

And the same IEnumerable is covariant. What are the .NET developers guided by in a similar question?

Example

class Program
    {
        static void Main(string[] args)
        {
            List<User> users = null;
            List<Administrator> admins = null;

            users = admins; // ошибка потому что List<T> не ковариантен
            IEnumerable<User> users2 = admins; // ошибки нет т.к. IEnumerable<out T> ковариантен
        }
    }

    public class User { }
    public class Administrator : User { }

Answer 1

They are not variant, because T is accepted as an input (Add), and returned in methods (Get).
And C# in such situations does not allow you to add the appropriate modifiers.
Here with IReadOnlyList there is no such problem because it is covariant.