Why doesn't an interface work but an abstract class does with a generic class constraint?

Question

The code below shows a generic class with a type constraint (Pub). The class has an event that it can raise allowing us to pass a message to subscriber

Answer 1

Replace IMsg with T

public interface IMsg { }        // Doesn't work

public class Msg : IMsg { }

public class Pub where T : IMsg
{
    public event Action notify;

    public void Subscribe(object subscriber)
    {
        IHandler implementer = subscriber as IHandler; // here

        if (implementer != null)
        {
            this.notify += implementer.NotifyEventHandler;
        }
    }
}

public interface IHandler where T : IMsg
{
    void NotifyEventHandler(T data);
}