Well as we all know that type of thing gets installed on a Tuesday.
But too feature it would cost money, a new CGI model of Voyager would have to be created or an existing one modified. a New CGI model of the Aeroshuttle would have to be created. Which all cost money.
The writers no doubt simply forgot it was there.
As for why the plug might have hull markings etc, perhaps to disguise the fact that it isn't there.