Or is it because business travel is down so far that the guys sitting alone in the hotel are no longer ordering it?
Could be. But enough people are traveling that Marriott's opening new hotels. The story is that they are phasing out the porn in the new hotels they'll be opening over the next three years. My guess is that the payment for the in-room porn is probably also based on a per-view basis. They're going to keep the movie units in place so people can watch regular movies - so it's just the matter of paying for the specially-cut Marriott porn.
Last sentence I wrote: "Or because it's no longer such a profit machine." The articles I linked mentioned that travelers often check in with laptops that have porn loaded, or with portable players, or Slingboxes.