Honestly, at this point I am inclined to believe that a pairing between Sheldon/Penny at a romantic level would be completely catastrophic to the show. Imagine Friends; Instead of Rachel/Ross and Phoebe/Mike, it would be Phoebe Ross with Rachel/Mike. Scrubs with Carla/JD and Turk/Elliot instead of Elliot/JD and Turk/Carla ,Charlie Harper paired with Judith or something along these lines. It's not that Shenny is a terrible ship, but if it were too hypothetically happen, the entire dynamic of the series would be so twisted that it would off put a lot of loyal, long time fans including myself and this seems to be the most logical explanation for the way the writers treated this pairing. Most of the time, in other series they wouldn't straight up reject it. The accepted procedure is to say something along the lines of "anything can happen" but to say it is straight up off their list shows how unrealistic this pairing really is six seasons into the series.