Why doesn't C # allow implicit reference conversions from interface type to class?

Given:

public class T 
{
    public static implicit operator T(R r)
    {
        return new T();
    }
}

public class S: R { }
public class R { }

      

        
        
        
      

    

If R is an interface, the code will not compile:

custom conversions to or from the interface are not allowed

Interfaces are essentially public / inner classes with all public members and no implementation. So what is the purpose of this limitation?