This is a personal opinion and cannot be backed up by facts...
I think it has a lot to do with how Ballroom is perceived in general. While those in the field know how rigorous it is, how demanding it is, and that those that do it are indeed athletes without an off-season, to the general public it is a hobby. A great deal of our technique goes into hiding what we do, and to an extent, it should; that's the same with almost all artistry. No one is supposed to see your work, just the results. And I think it's because of that perception Ballroom isn't offered as an academic course, or is treated with the same seriousness in academic circles as say Classical Ballet or Modern. There is a lot more tradition in Ballet, a lot more context and perception of it being as difficult, physically demanding, grueling. I would argue that the top Ballet dancer and the top Ballroom dancer are in the same level of bodily awareness. They worked just as hard to gain mastery. They just use it differently.
I'm not sure if that explanation is clear, but that's my feeling. I'm trying hard to stay on topic and not go on a rant about something else that seems related but really isn't. If enough information was available about Ballroom and people understood that it is just as demanding and rigorous, they may be less inclined to write it off like they do.
I am also inclined to agree with bia's post. It's fairly spot on, if I had to pick a second explanation.
